Segfault in qv4value_inl_p.h:52 isString due to commit e7b66a60584b3fcb1436a7dbe8396b71cb69fe1b

After upgrading from Qt 5.4.1 to the git of the 5.5 branch (I did that to get access to fixes to QScreen... in XCB QPA to avoid crashes when I switch screens), my KDE Plasma 5.2.1 keeps crashing:

#6 0x00007f6473fa3b85 in isString (this=0x7f6428ffe078) at /var/tmp/portage/dev-qt/qtdeclarative-5.5.9999/work/qtdeclarative-5.5.9999/src/qml/jsruntime/qv4value_inl_p.h:52
#7 QV4::RuntimeHelpers::equalHelper (x=..., y=...) at /var/tmp/portage/dev-qt/qtdeclarative-5.5.9999/work/qtdeclarative-5.5.9999/src/qml/jsruntime/qv4runtime.cpp:708
#8 0x00007f64384db54d in ?? ()
#9 0x00007f6478916700 in ?? ()
#10 0x0001800000000000 in ?? ()
#11 0x0003000000000003 in ?? ()
#12 0x00007f647866c9d0 in ?? ()
#13 0x00007f642a800100 in ?? ()
#14 0x00007f642a800100 in ?? ()
#15 0x00007f64786f7970 in ?? ()
#16 0x00007f6428ffe068 in ?? ()
#17 0x00007fffbef81cd0 in ?? ()
#18 0x00007f6473f447c2 in QV4::SimpleScriptFunction::call (that=<optimized out>, callData=<optimized out>) at /var/tmp/portage/dev-qt/qtdeclarative-5.5.9999/work/qtdeclarative-5.5.9999/src/qml/jsruntime/qv4functionobject.cpp:539
Backtrace stopped: frame did not save the PC

See the attached gdb backtraces and reports from valgrind. `git bisect` identified a first failing commit, e7b66a60584b3fcb1436a7dbe8396b71cb69fe1b ("Make Managed inherit Value").