Crash in QWaylandCursor depending on ordering of interfaces

Depending on the ordering of the interfaces in the Wayland server QWaylandCursor might crash on application startup. This happens if the server announces wl_output before wl_shm. I am able to trigger this with a KWayland demo server.

E.g. with weston-info returning the following we get a crash:
interface: 'wl_compositor', version: 3, name: 1
interface: 'wl_shell', version: 1, name: 2
interface: 'wl_output', version: 2, name: 3
x: 0, y: 0,
physical_width: 10 mm, physical_height: 10 mm,
make: 'org.kde.kwin', model: 'none',
subpixel_orientation: unknown, output_transform: normal,
mode:
width: 1024 px, height: 768 px, refresh: 60 Hz,
flags: current
interface: 'wl_shm', version: 1, name: 4
formats: XRGB8888 ARGB8888

While with weston-info returning the following we don't get a crash:
interface: 'wl_shm', version: 1, name: 1
formats: XRGB8888 ARGB8888
interface: 'wl_compositor', version: 3, name: 2
interface: 'wl_shell', version: 1, name: 3
interface: 'wl_output', version: 2, name: 4
x: 0, y: 0,
physical_width: 10 mm, physical_height: 10 mm,
make: 'org.kde.kwin', model: 'none',
subpixel_orientation: unknown, output_transform: normal,
mode:
width: 1024 px, height: 768 px, refresh: 60 Hz,
flags: current

The backtrace of the crash if wl_output is announced before wl_shm:
Program received signal SIGSEGV, Segmentation fault.
0x00007fffef167ea6 in wl_proxy_marshal_constructor (proxy=0x0, opcode=0, interface=0x7ffff62aea60 <wl_shm_pool_interface>) at ../src/wayland-client.c:625
625 ../src/wayland-client.c: No such file or directory.
(gdb) bt
#0 0x00007fffef167ea6 in wl_proxy_marshal_constructor (proxy=0x0, opcode=0, interface=0x7ffff62aea60 <wl_shm_pool_interface>) at ../src/wayland-client.c:625
#1 0x00007fffeef5b543 in wl_cursor_theme_load () from /usr/lib/x86_64-linux-gnu/libwayland-cursor.so.0
#2 0x00007ffff7f38270 in QWaylandCursor::QWaylandCursor (this=0x61a9d0, screen=0x61edb0) at qwaylandcursor.cpp:68
#3 0x00007ffff7f45635 in QWaylandScreen::QWaylandScreen (this=0x61edb0, waylandDisplay=0x619cb0, version=2, id=3) at qwaylandscreen.cpp:68
#4 0x00007ffff7f3cd2f in QWaylandDisplay::registry_global (this=0x619cb0, id=3, interface=..., version=2) at qwaylanddisplay.cpp:232
#5 0x00007ffff7f54da5 in QtWayland::wl_registry::handle_global (data=0x619cc0, object=0x6134a0, name=3, interface=0x7fffe8007a20 "wl_output", version=2) at qwayland-wayland.cpp:67
#6 0x00007ffff2a63dc0 in ffi_call_unix64 () from /usr/lib/x86_64-linux-gnu/libffi.so.6
#7 0x00007ffff2a63828 in ffi_call () from /usr/lib/x86_64-linux-gnu/libffi.so.6
#8 0x00007fffef16a52d in wl_closure_invoke (closure=closure@entry=0x7fffe8007940, flags=flags@entry=1, target=target@entry=0x6134a0, opcode=opcode@entry=0, data=0x619cc0) at ../src/connection.c:934
#9 0x00007fffef16786b in dispatch_event (display=display@entry=0x7fffe8003320, queue=0x6127b0) at ../src/wayland-client.c:1136
#10 0x00007fffef1678ec in dispatch_queue (display=display@entry=0x7fffe8003320, queue=queue@entry=0x6127b0) at ../src/wayland-client.c:1261
#11 0x00007fffef1685f4 in wl_display_dispatch_queue_pending (display=0x7fffe8003320, queue=0x6127b0) at ../src/wayland-client.c:1479
#12 0x00007ffff7f3c745 in QWaylandDisplay::flushRequests (this=0x619cb0) at qwaylanddisplay.cpp:174
#13 0x00007ffff7f5b8aa in QWaylandDisplay::qt_static_metacall (_o=0x619cb0, _c=QMetaObject::InvokeMetaMethod, _id=1, _a=0x7fffe80075c0) at .moc/moc_qwaylanddisplay_p.cpp:73
#14 0x00007ffff71e4d0f in QMetaCallEvent::placeMetaCall (this=0x7fffe8007c30, object=0x619cb0) at kernel/qobject.cpp:485
#15 0x00007ffff71e5dd8 in QObject::event (this=0x619cb0, e=0x7fffe8007c30) at kernel/qobject.cpp:1245
#16 0x00007ffff71a97d0 in QCoreApplicationPrivate::notify_helper (this=0x60ef80, receiver=0x619cb0, event=0x7fffe8007c30) at kernel/qcoreapplication.cpp:1052
#17 0x00007ffff71a94b2 in QCoreApplication::notify (this=0x7fffffffdcd0, receiver=0x619cb0, event=0x7fffe8007c30) at kernel/qcoreapplication.cpp:997
#18 0x00007ffff71a93bc in QCoreApplication::notifyInternal (this=0x7fffffffdcd0, receiver=0x619cb0, event=0x7fffe8007c30) at kernel/qcoreapplication.cpp:935
#19 0x00007ffff71ad01d in QCoreApplication::sendEvent (receiver=0x619cb0, event=0x7fffe8007c30) at ../../include/QtCore/../../src/corelib/kernel/qcoreapplication.h:228
#20 0x00007ffff71aa6f5 in QCoreApplicationPrivate::sendPostedEvents (receiver=0x0, event_type=0, data=0x607740) at kernel/qcoreapplication.cpp:1552
#21 0x00007ffff71aa0a4 in QCoreApplication::sendPostedEvents (receiver=0x0, event_type=0) at kernel/qcoreapplication.cpp:1410
#22 0x00007ffff7221ee2 in postEventSourceDispatch (s=0x6074e0) at kernel/qeventdispatcher_glib.cpp:271
#23 0x00007ffff30dbc5d in g_main_context_dispatch () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#24 0x00007ffff30dbf48 in ?? () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#25 0x00007ffff30dbffc in g_main_context_iteration () from /lib/x86_64-linux-gnu/libglib-2.0.so.0
#26 0x00007ffff722269d in QEventDispatcherGlib::processEvents (this=0x6043a0, flags=...) at kernel/qeventdispatcher_glib.cpp:418
#27 0x00007ffff7f3dbbd in QWaylandDisplay::forceRoundTrip (this=0x619cb0) at qwaylanddisplay.cpp:344
#28 0x00007ffff7f3c4c4 in QWaylandDisplay::QWaylandDisplay (this=0x619cb0, waylandIntegration=0x613050) at qwaylanddisplay.cpp:159
#29 0x00007ffff7f2ec71 in QWaylandIntegration::QWaylandIntegration (this=0x613050) at qwaylandintegration.cpp:127
#30 0x00007ffff7ff144a in QWaylandIntegrationPlugin::create (this=0x612ad0, system=..., paramList=...) at main.cpp:59
#31 0x00007ffff759db3d in QPlatformIntegrationPlugin::create (this=0x612ad0, key=..., paramList=..., argc=@0x7fffffffdc4c: 1, argv=0x7fffffffdec8) at kernel/qplatformintegrationplugin.cpp:58
#32 0x00007ffff759cf72 in loadIntegration (loader=0x7ffff7b6e520 <ZZN12_GLOBAL_N_112Q_QGS_loader13innerFunctionEvE6holder>, key=..., parameters=..., argc=@0x7fffffffdc4c: 1, argv=0x7fffffffdec8)
at kernel/qplatformintegrationfactory.cpp:56
#33 0x00007ffff759d02a in QPlatformIntegrationFactory::create (platform=..., paramList=..., argc=@0x7fffffffdc4c: 1, argv=0x7fffffffdec8, platformPluginPath=...) at kernel/qplatformintegrationfactory.cpp:73
#34 0x00007ffff75a8adb in init_platform (pluginArgument=..., platformPluginPath=..., platformThemeName=..., argc=@0x7fffffffdc4c: 1, argv=0x7fffffffdec8) at kernel/qguiapplication.cpp:1010
#35 0x00007ffff75a9d0e in QGuiApplicationPrivate::createPlatformIntegration (this=0x60ef80) at kernel/qguiapplication.cpp:1165
#36 0x00007ffff75a9efd in QGuiApplicationPrivate::eventDispatcherReady (this=0x60ef80) at kernel/qguiapplication.cpp:1193
#37 0x00007ffff71a906b in QCoreApplication::init (this=0x7fffffffdcd0) at kernel/qcoreapplication.cpp:739
#38 0x00007ffff71a8d55 in QCoreApplication::QCoreApplication (this=0x7fffffffdcd0, p=...) at kernel/qcoreapplication.cpp:655
#39 0x00007ffff75a7879 in QGuiApplication::QGuiApplication (this=0x7fffffffdcd0, argc=@0x7fffffffdc4c: 1, argv=0x7fffffffdec8, flags=328706) at kernel/qguiapplication.cpp:548
#40 0x0000000000402917 in main (argc=1, argv=0x7fffffffdec8) at /home/martin/src/kf5/kde/workspace/kwayland/tests/waylandservertest.cpp:123