GVariant: fix string validation

GVariant: fix string validation

String validation was done by checking if the string was valid utf8 and
ensuring that the first non-utf8 character was the last character (ie:
the nul terminator).

No check was actually done to make sure that this byte actually
contained a nul, however, so it was possible that you could have a
string like "hello\xff" with length 6 that would correctly validate.

Fix that, and test it.