avcodec/h264_slice: update slice context linesizes when a new picture is allocated

previously they where only updated when decoding started and would thus have
been inconsistent for a longer period of time leaving more chances for
problems
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/h264_slice.c

@@ -711,6 +711,11 @@ static int h264_frame_start(H264Context *h)
     if ((ret = ff_h264_ref_picture(h, &h->cur_pic, h->cur_pic_ptr)) < 0)
         return ret;
 
+    for (i = 0; i < h->nb_slice_ctx; i++) {
+        h->slice_ctx[i].linesize   = h->cur_pic_ptr->f.linesize[0];
+        h->slice_ctx[i].uvlinesize = h->cur_pic_ptr->f.linesize[1];
+    }
+
     if (CONFIG_ERROR_RESILIENCE) {
         ff_er_frame_start(&h->slice_ctx[0].er);
         ff_h264_set_erpic(&h->slice_ctx[0].er.last_pic, NULL);