avcodec: disallow reget_buffer() if pix_fmt changed.

libavcodec/utils.c

@@ -553,6 +553,8 @@ int avcodec_default_reget_buffer(AVCodecContext *s, AVFrame *pic){
         return s->get_buffer(s, pic);
     }
 
+    assert(s->pix_fmt == pic->pix_fmt);
+
     /* If internal buffer type return the same buffer */
     if(pic->type == FF_BUFFER_TYPE_INTERNAL) {
         if(s->pkt) pic->pkt_pts= s->pkt->pts;