Attempting to use an invalid QStringConverter would so far have resulted
in a crash, as we would dereference the null iface pointer.
Fix this by inserting adequate checks, and ensure that hasError returns
true if we attempt to en/decode with an invalid converter.
Pick-to: 6.2 6.3
Change-Id: Icf74bb88cd8c95685481cc0bd512da99b62f33e6
Reviewed-by: Lars Knoll <lars.knoll@qt.io>