gvariant: Fix bounds checking in GVariant text format parser

gvariant: Fix bounds checking in GVariant text format parser

The token_stream_peek() functions were not doing any bounds checking, so
could potentially read 1 byte off the end of the input blob. This was
never noticed, since the input stream is almost always a nul-terminated
string. However, g_variant_parse() does allow non-nul-terminated strings
to be used with a @limit parameter, and the bugs become apparent under
valgrind if that parameter is used.

This includes modifications to the test cases to cover the
non-nul-terminated case.

Spotted by ossfuzz.

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