h263: more strictly forbid frame size changes with frame-mt.

Prevents crashes because the old check was incomplete.

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org

libavcodec/h263dec.c

@@ -430,6 +430,13 @@ retry:
     if (ret < 0){
         av_log(s->avctx, AV_LOG_ERROR, "header damaged\n");
         return -1;
+    } else if ((s->width  != avctx->coded_width  ||
+                s->height != avctx->coded_height ||
+                (s->width  + 15) >> 4 != s->mb_width ||
+                (s->height + 15) >> 4 != s->mb_height) &&
+               (HAVE_THREADS && (s->avctx->active_thread_type & FF_THREAD_FRAME))) {
+        av_log_missing_feature(s->avctx, "Width/height/bit depth/chroma idc changing with threads is", 0);
+        return AVERROR_PATCHWELCOME;   // width / height changed during parallelized decoding
     }
 
     avctx->has_b_frames= !s->low_delay;
@@ -571,11 +578,6 @@ retry:
         /* H.263 could change picture size any time */
         ParseContext pc= s->parse_context; //FIXME move these demuxng hack to avformat
 
-        if (HAVE_THREADS && (s->avctx->active_thread_type&FF_THREAD_FRAME)) {
-            av_log_missing_feature(s->avctx, "Width/height/bit depth/chroma idc changing with threads is", 0);
-            return -1;   // width / height changed during parallelized decoding
-        }
-
         s->parse_context.buffer=0;
         ff_MPV_common_end(s);
         s->parse_context= pc;