avcodec/sgidec: fix linesize for 16bit

Fixes: asan_heap-oob_22b30d4_39_038.sgi
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/sgidec: fix count check

Fixes: asan_heap-oob_22b30d4_39_038.sgi
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>