We weren't doing the decoding from UTF-16 to UTF-32, so weren't catching
invalid code sequences
[ChangeLog][QtCore][QXmlStreamWriter] The class now rejects writing
UTF-8 and UTF-16 invalid input (improper code unit sequences).
Task-number: QTBUG-122241
Pick-to: 6.5 6.6 6.7
Change-Id: I83dda2d36c904517b3c0fffd17b42d17c637fdc4
Reviewed-by: Mate Barany <mate.barany@qt.io>