When validating a string to see if it’s valid UTF-8, we pass a gsize to
g_utf8_validate(), which only takes a gssize. For large gsize values,
this will result in the gssize actually being negative, which will
change g_utf8_validate()’s behaviour to stop at the first nul byte. That
would allow subsequent nul bytes through the string validator, against
its documented behaviour.

Add a test case.

oss-fuzz#10319

Signed-off-by: Philip Withnall <withnall@endlessm.com>