avcodec/utvideodec: Fix handling of slice_height=0

Fixes out of array accesses
Fixes: asan_heap-oob_25bcd7e_3783_cov_3553517262_utvideo_rgba_median.avi
Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
