AddressSanitizer: attempting free on address which was not malloc()-ed - in WebEngine

It looks like if there is a misplaced free in webengine.

Reproduction steps:
0.1. Start demobrowser w/o Address Sanitizer
0.2. Enter Proxy Data (Alternatively make sure, system proxy is properly set up)
0.3. Quit again
1. Build with Address Sanitizer and start

demobrowser$ cat >>demobrowser.pro <<EOF
>
> QMAKE_CC = clang
> QMAKE_CXX = clang++
>
> QMAKE_LINK = $$QMAKE_CXX
> QMAKE_LINK_SHLIB = $$QMAKE_CXX
>
> QMAKE_CXXFLAGS += -Qunused-arguments -Wno-tautological-undefined-compare -Wno-undefined-bool-conversion -Wno-deprecated-register -Wno-mismatched-tags
> QMAKE_CFLAGS += -Qunused-arguments
>
> QMAKE_LFLAGS += -fsanitize=address
> QMAKE_CXXFLAGS += -O1 -fsanitize=address -fno-omit-frame-pointer
>
> EOF
demobrowser$ $QTDIR/bin/qmake && make && ./demobrowser 'https://www.qt.io/'
[...]

2. Enter proxy username & Password
3. Watch it crash

[...]
QStandardPaths: XDG_RUNTIME_DIR not set, defaulting to '/tmp/runtime-m2puser'
qt.network.ssl: QSslSocket: cannot resolve SSLv2_client_method
qt.network.ssl: QSslSocket: cannot resolve SSLv2_server_method
=================================================================
==19580==ERROR: AddressSanitizer: attempting free on address which was not malloc()-ed: 0x000001fc6010 in thread T0 (demobrowser)
#0 0x4ed060 (/home/bernd/devel/demobrowser/demobrowser+0x4ed060)
#1 0x7f2cfc2877cc (/opt/Qt5.9.1/5.9.1/gcc_64/lib/libQt5WebEngineCore.so.5+0x11657cc)
#2 0x7f2cfc287818 (/opt/Qt5.9.1/5.9.1/gcc_64/lib/libQt5WebEngineCore.so.5+0x1165818)
#3 0x7f2cfc1fec3b (/opt/Qt5.9.1/5.9.1/gcc_64/lib/libQt5WebEngineCore.so.5+0x10dcc3b)
#4 0x7f2cfc1feed8 (/opt/Qt5.9.1/5.9.1/gcc_64/lib/libQt5WebEngineCore.so.5+0x10dced8)
#5 0x7f2cfbec7c53 (/opt/Qt5.9.1/5.9.1/gcc_64/lib/libQt5WebEngineCore.so.5+0xda5c53)
#6 0x7f2cfc9fc2f5 (/opt/Qt5.9.1/5.9.1/gcc_64/lib/libQt5WebEngineCore.so.5+0x18da2f5)
#7 0x7f2cfca00b4a (/opt/Qt5.9.1/5.9.1/gcc_64/lib/libQt5WebEngineCore.so.5+0x18deb4a)
#8 0x7f2cfca01e73 (/opt/Qt5.9.1/5.9.1/gcc_64/lib/libQt5WebEngineCore.so.5+0x18dfe73)
#9 0x7f2cfc9f9dce (/opt/Qt5.9.1/5.9.1/gcc_64/lib/libQt5WebEngineCore.so.5+0x18d7dce)
#10 0x7f2cfc9f9eed (/opt/Qt5.9.1/5.9.1/gcc_64/lib/libQt5WebEngineCore.so.5+0x18d7eed)
#11 0x7f2cfca06dc4 (/opt/Qt5.9.1/5.9.1/gcc_64/lib/libQt5WebEngineCore.so.5+0x18e4dc4)
#12 0x7f2cfc1e41c8 (/opt/Qt5.9.1/5.9.1/gcc_64/lib/libQt5WebEngineCore.so.5+0x10c21c8)
#13 0x7f2cfc18899f (/opt/Qt5.9.1/5.9.1/gcc_64/lib/libQt5WebEngineCore.so.5+0x106699f)
#14 0x7f2cfc18a5dc (/opt/Qt5.9.1/5.9.1/gcc_64/lib/libQt5WebEngineCore.so.5+0x10685dc)
#15 0x7f2cfc18aa94 (/opt/Qt5.9.1/5.9.1/gcc_64/lib/libQt5WebEngineCore.so.5+0x1068a94)
#16 0x7f2cfb8480b4 (/opt/Qt5.9.1/5.9.1/gcc_64/lib/libQt5WebEngineCore.so.5+0x7260b4)
#17 0x7f2cf860c37a (/opt/Qt5.9.1/5.9.1/gcc_64/lib/libQt5Core.so.5+0x2a437a)
#18 0x7f2cfa1a34bb (/opt/Qt5.9.1/5.9.1/gcc_64/lib/libQt5Widgets.so.5+0x1534bb)
#19 0x7f2cfa1aa8e6 (/opt/Qt5.9.1/5.9.1/gcc_64/lib/libQt5Widgets.so.5+0x15a8e6)
#20 0x7f2cf85e0eb7 (/opt/Qt5.9.1/5.9.1/gcc_64/lib/libQt5Core.so.5+0x278eb7)
#21 0x7f2cf85e354a (/opt/Qt5.9.1/5.9.1/gcc_64/lib/libQt5Core.so.5+0x27b54a)
#22 0x7f2cf8633ad2 (/opt/Qt5.9.1/5.9.1/gcc_64/lib/libQt5Core.so.5+0x2cbad2)
#23 0x7f2cf6256196 (/lib/x86_64-linux-gnu/libglib-2.0.so.0+0x4a196)
#24 0x7f2cf62563ef (/lib/x86_64-linux-gnu/libglib-2.0.so.0+0x4a3ef)
#25 0x7f2cf625649b (/lib/x86_64-linux-gnu/libglib-2.0.so.0+0x4a49b)
#26 0x7f2cf863311e (/opt/Qt5.9.1/5.9.1/gcc_64/lib/libQt5Core.so.5+0x2cb11e)
#27 0x7f2cf85df4a9 (/opt/Qt5.9.1/5.9.1/gcc_64/lib/libQt5Core.so.5+0x2774a9)
#28 0x7f2cf85e7d33 (/opt/Qt5.9.1/5.9.1/gcc_64/lib/libQt5Core.so.5+0x27fd33)
#29 0x603897 (/home/bernd/devel/demobrowser/demobrowser+0x603897)
#30 0x7f2cf6da882f (/lib/x86_64-linux-gnu/libc.so.6+0x2082f)
#31 0x44d0b8 (/home/bernd/devel/demobrowser/demobrowser+0x44d0b8)

AddressSanitizer can not describe address in more detail (wild memory access suspected).
SUMMARY: AddressSanitizer: bad-free (/home/bernd/devel/demobrowser/demobrowser+0x4ed060)
==19580==ABORTING
demobrowser$