avcodec/g2meet: fix stride calculation, use correct format field

Fixes out of array accesses
Fixes: asan_heap-oob_ae5f63_5415_g2m4.wmv
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
