During a kconf_update run, an invalid group name may be treated as correct
even though the name failed to unescape.  This leads the group name to be
a null character, which will fail.

If the unescape failed, return that failure status instead.  This should have
no impact, as the previous result would have been wrong anyways.  Now a
more useful diagnostic will be returned instead.

Update the unit tests to ensure this issue is tested in the future.

Found in Coverity issue 258087.

REVIEW: 126556

Due to https://codereview.qt-project.org/#/c/106473/, Qt 5.6 keeps null
characters in QByteArray -> QString conversions, which breaks this test as
one QByteArray contains nulls.  Instead, convert the QByteArray to const
char * first, so QString stops at the first null.

The actual  behaviour of KConfig is unchanged, as internally the conversion
always went through a const char *, which avoids creating QStrings with
null characters.

REVIEW: 126001

REVIEW: 125833

Fallback will be :/kconfig/
REVIEW: 125598

CHANGELOG: KDesktopFile can now read mime types specified in the MimeType field

REVIEW: 124746

Qt 5.5 writes out "123.567001" for 123.567f. No matter, what we want is the
roundtrip float-string-float to work, and it does.

to avoid touching the user's real files