utils: check that parameters havnt changed in reget_buffer()

Fixes Ticket902
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/utils.c

@@ -557,6 +557,11 @@ int avcodec_default_reget_buffer(AVCodecContext *s, AVFrame *pic){
 
     assert(s->codec_type == AVMEDIA_TYPE_VIDEO);
 
+    if (pic->data[0] && (pic->width != s->width || pic->height != s->height || pic->format != s->pix_fmt)) {
+        av_log(s, AV_LOG_WARNING, "Width/height/fmt changing with reget buffer");
+        s->release_buffer(s, pic);
+    }
+
     /* If no picture return a new buffer */
     if(pic->data[0] == NULL) {
         /* We will copy from buffer, so must be readable */