Always reset slice_table.

Previously, the area of a lost slice would be left at the slice number of the previous
frame which could occasionally match the number of the next slice and thus a non existing
slice could have been used for prediction leading to additional decoding errors in otherwise
undamaged slices.

Originally committed as revision 22483 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/h264.c

@@ -961,8 +961,7 @@ int ff_h264_frame_start(H264Context *h){
         if(!h->thread_context[i]->s.obmc_scratchpad)
             h->thread_context[i]->s.obmc_scratchpad = av_malloc(16*2*s->linesize + 8*2*s->uvlinesize);
 
-    /* some macroblocks will be accessed before they're available */
-    if(FRAME_MBAFF || s->avctx->thread_count > 1)
+    /* some macroblocks can be accessed before they're available in case of lost slices, mbaff or threading*/
         memset(h->slice_table, -1, (s->mb_height*s->mb_stride-1) * sizeof(*h->slice_table));
 
 //    s->decode= (s->flags&CODEC_FLAG_PSNR) || !s->encoding || s->current_picture.reference /*|| h->contains_intra*/ || 1;