avcodec/h264_slice: Use w/h from the AVFrame instead of mb_w/h

Fixes out of array access
Fixes: asan_heap-oob_4d5bb0_682_cov_3124593265_Fraunhofer__a_driving_force_in_innovation__small.mp4

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