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.
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