Fix crash in QDBusDemarshaller basic string-like type extraction

Fix crash in QDBusDemarshaller basic string-like type extraction

QDBusArgument string extraction operators and QDBusDemarshaller that
implements the extraction do not check the type of the extracted value.
When extracting string-like basic DBus type that actually is e.g. an
integer the string extraction will crash as it blindly attempts to use the
integer as a pointer to char.

The fix adds DBus type checks to QDBusArgument string type extraction
operator implementations.
The checks are as permissive as possible provided crashes are avoided.
Previously supported functionality of extracting an object path or type
signature to a string type is retained.

Task-number: QTBUG-22840
Change-Id: I29be1ae592658ca268c65ed692e1d42619d52280
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>