gvariant: Fix more bounds checking in GVariant text format parser
token_stream_prepare() was over-reading at the start of bytestring
literals (b'blah'
).
Add tests for that, and for some other situations regarding bytestring literal parsing, in order to try and get full branch coverage of that bit of code.
oss-fuzz#9805
Signed-off-by: Philip Withnall withnall@endlessm.com