The fuzzer will produce arbitrary binary blobs, which might not be
nul-terminated. `g_uri_parse()` has no length argument, so relies on
receiving a nul-terminated string as input. Guarantee that.
This should fix fuzzing build failures like
https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=23750.
Signed-off-by: Philip Withnall <withnall@endlessm.com>