vp9: avoid infinite loop with broken files

With a certain fuzzed file, the parser will always return 0 consumed
bytes, which makes calling code call the parser infinitely. Return the
full packet size on error instead. (Here it would be nice if parsers
could return errors at all.)

Additionally, _if_ there's some data left, return that too, which might
help with somewhat broken but still somehow playable files.

Fixes ticket #4242.

Reviewed-by: "Ronald S. Bultje" <rsbultje@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>