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>

libavcodec/h264_slice.c

@@ -1579,8 +1579,8 @@ int ff_h264_decode_slice_header(H264Context *h, H264SliceContext *sl)
                                   (const uint8_t **)prev->f->data,
                                   prev->f->linesize,
                                   prev->f->format,
-                                  h->mb_width  * 16,
-                                  h->mb_height * 16);
+                                  prev->f->width,
+                                  prev->f->height);
                     h->short_ref[0]->poc = prev->poc + 2;
                 }
                 h->short_ref[0]->frame_num = h->prev_frame_num;