Investigate potential gcc bug in QPropertyObserverPointer::notify

Without a seemingly superfluous 

char preventBug[1] = {'\0'};

line, we get a crash in tst_qquickitem2 childrenRect with 10.2.0 in an ASAN debug build, if QQuickItem::x/y/width/height are ported to the QProperty system. With clang, the issue do es not exist.