avcodec/rawdec: Check the return code of avpicture_get_size()

Fixes out of array access
Fixes: asan_heap-oob_22388d0_3435_cov_3297128910_small_roll5_FlashCine1.cine
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
