avcodec/h264_slice: Fix container cropping

Fixes out of array read
Fixes: asan_heap-oob_394322e_138_cov_4265020547_CVPCMNL1_SVA_C.264

Found-by: Samuel Groß, Mateusz "j00ru" Jurczyk and Gynvael Coldwind
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
