Crash in qqmlenginedebug service autotest

Seen in e.g. https://codereview.qt-project.org/#change,39089

Testing tst_QQmlEngineDebugService
Totals: 35 passed, 0 failed, 0 skipped
QtQA::App::TestRunner: Process exited due to signal 11; dumped core
QtQA::App::TestRunner: ============================== backtrace follows: ==============================
QtQA::App::TestRunner: gdb commands: thread apply all bt 100
QtQA::App::TestRunner:
QtQA::App::TestRunner: warning: core file may not match specified executable file.
QtQA::App::TestRunner: [New LWP 28343]
QtQA::App::TestRunner: [New LWP 28345]
QtQA::App::TestRunner:
QtQA::App::TestRunner: warning: Can't read pathname for load map: Input/output error.
QtQA::App::TestRunner: [Thread debugging using libthread_db enabled]
QtQA::App::TestRunner: Core was generated by `./tst_qqmlenginedebugservice -silent -o /work/build/_artifacts/test-logs/tst_qq'.
QtQA::App::TestRunner: Program terminated with signal 11, Segmentation fault.
QtQA::App::TestRunner: #0 QObject::~QObject (this=0x8d46250, __in_chrg=<optimized out>) at /work/build/qt/qtbase/src/corelib/kernel/qobject.cpp:753
QtQA::App::TestRunner: 753 d->wasDeleted = true;
QtQA::App::TestRunner:
QtQA::App::TestRunner: Thread 2 (Thread 0xb6844b70 (LWP 28345)):
QtQA::App::TestRunner: #0 0xb775d424 in __kernel_vsyscall ()
QtQA::App::TestRunner: #1 0xb60ffd35 in sem_wait@@GLIBC_2.1 () at ../nptl/sysdeps/unix/sysv/linux/i386/i686/../i486/sem_wait.S:80
QtQA::App::TestRunner: #2 0xb645a326 in v8::internal::LinuxSemaphore::Wait (this=0x8cd7108) at /work/build/qt/qtjsbackend/src/3rdparty/v8/src/platform-linux.cc:857
QtQA::App::TestRunner: #3 0xb6389e98 in v8::internal::RuntimeProfiler::WaitForSomeIsolateToEnterJS () at /work/build/qt/qtjsbackend/src/3rdparty/v8/src/runtime-profiler.cc:420
QtQA::App::TestRunner: #4 0xb645bd3b in v8::internal::SignalSender::Run (this=0x8cd73e8) at /work/build/qt/qtjsbackend/src/3rdparty/v8/src/platform-linux.cc:1136
QtQA::App::TestRunner: #5 0xb645a3db in v8::internal::ThreadEntry (arg=0x8cd73e8) at /work/build/qt/qtjsbackend/src/3rdparty/v8/src/platform-linux.cc:733
QtQA::App::TestRunner: #6 0xb60f9d31 in start_thread (arg=0xb6844b70) at pthread_create.c:304
QtQA::App::TestRunner: #7 0xb667f80e in clone () at ../sysdeps/unix/sysv/linux/i386/clone.S:130
QtQA::App::TestRunner: Backtrace stopped: Not enough registers or memory available to unwind further
QtQA::App::TestRunner:
QtQA::App::TestRunner: Thread 1 (Thread 0xb4c756e0 (LWP 28343)):
QtQA::App::TestRunner: #0 QObject::~QObject (this=0x8d46250, __in_chrg=<optimized out>) at /work/build/qt/qtbase/src/corelib/kernel/qobject.cpp:753
QtQA::App::TestRunner: #1 0xb6a93cd2 in QObject::~QObject (this=0x8d46250, __in_chrg=<optimized out>) at /work/build/qt/qtbase/src/corelib/kernel/qobject.cpp:863
QtQA::App::TestRunner: #2 0xb6d8f366 in QFontCache::clear (this=0x8d0ec80) at /work/build/qt/qtbase/src/gui/text/qfont.cpp:2701
QtQA::App::TestRunner: #3 0xb6d8f589 in QFontCache::~QFontCache (this=0x8d0ec80, __in_chrg=<optimized out>) at /work/build/qt/qtbase/src/gui/text/qfont.cpp:2638
QtQA::App::TestRunner: #4 0xb6d8f732 in QFontCache::~QFontCache (this=0x8d0ec80, __in_chrg=<optimized out>) at /work/build/qt/qtbase/src/gui/text/qfont.cpp:2669
QtQA::App::TestRunner: #5 0xb6d901d3 in qThreadStorage_deleteData<QFontCache> (d=0x8d0ec80) at ../../include/QtCore/../../../../../../work/build/qt/qtbase/src/corelib/thread/qthreadstorage.h:96
QtQA::App::TestRunner: #6 QThreadStorage<QFontCache*>::deleteData (x=0x8d0ec80) at ../../include/QtCore/../../../../../../work/build/qt/qtbase/src/corelib/thread/qthreadstorage.h:139
QtQA::App::TestRunner: #7 0xb68ba22d in QThreadStorageData::set (this=0x8d0da38, p=0x0) at /work/build/qt/qtbase/src/corelib/thread/qthreadstorage.cpp:165
QtQA::App::TestRunner: #8 0xb6d89d9d in qThreadStorage_setLocalData<QFontCache> (d=..., t=<optimized out>) at ../../include/QtCore/../../../../../../work/build/qt/qtbase/src/corelib/thread/qthreadstorage.h:91
QtQA::App::TestRunner: #9 setLocalData (t=0x0, this=0x8d0da38) at ../../include/QtCore/../../../../../../work/build/qt/qtbase/src/corelib/thread/qthreadstorage.h:154
QtQA::App::TestRunner: #10 QFontCache::cleanup () at /work/build/qt/qtbase/src/gui/text/qfont.cpp:2626
QtQA::App::TestRunner: #11 0xb6e67fd4 in QFont::cleanup () at /work/build/qt/qtbase/src/gui/text/qfont_qpa.cpp:54
QtQA::App::TestRunner: #12 0xb6d094f3 in QGuiApplicationPrivate::~QGuiApplicationPrivate (this=0x8c8a690, __in_chrg=<optimized out>) at /work/build/qt/qtbase/src/gui/kernel/qguiapplication.cpp:948
QtQA::App::TestRunner: #13 0xb6d09642 in QGuiApplicationPrivate::~QGuiApplicationPrivate (this=0x8c8a690, __in_chrg=<optimized out>) at /work/build/qt/qtbase/src/gui/kernel/qguiapplication.cpp:969
QtQA::App::TestRunner: #14 0xb6a93ba3 in cleanup (pointer=<optimized out>) at ../../include/QtCore/../../../../../../work/build/qt/qtbase/src/corelib/tools/qscopedpointer.h:63
QtQA::App::TestRunner: #15 ~QScopedPointer (this=<optimized out>, __in_chrg=<optimized out>) at ../../include/QtCore/../../../../../../work/build/qt/qtbase/src/corelib/tools/qscopedpointer.h:99
QtQA::App::TestRunner: #16 QObject::~QObject (this=0xbfeb1244, __in_chrg=<optimized out>) at /work/build/qt/qtbase/src/corelib/kernel/qobject.cpp:750
QtQA::App::TestRunner: #17 0xb6a66123 in QCoreApplication::~QCoreApplication (this=0xbfeb1244, __in_chrg=<optimized out>) at /work/build/qt/qtbase/src/corelib/kernel/qcoreapplication.cpp:643
QtQA::App::TestRunner: #18 0xb6d0977a in QGuiApplication::~QGuiApplication (this=0xbfeb1244, __in_chrg=<optimized out>) at /work/build/qt/qtbase/src/gui/kernel/qguiapplication.cpp:362
QtQA::App::TestRunner: #19 0x0804f0b3 in main (argc=6, argv=0xbfeb1324) at /work/build/qt/qtdeclarative/tests/auto/qml/debugger/qqmlenginedebugservice/tst_qqmlenginedebugservice.cpp:1198
QtQA::App::TestRunner:
QtQA::App::TestRunner: ================================================================================
QtQA::App::TestRunner: test failed, running again to see if it is flaky...