avcodec/g2meet: fix error returns

Fixes out of array accesses

This should not affect any release

Fixes: 8ab69af9e5a7a7e20fe04cdd25c0d6e7-asan_heap-oob_e72b82_5505_cov_2278389485_g2m4.wmv
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
