Reset prev_frame_num for idr frames as required by the spec.

Originally committed as revision 14354 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/h264.c

@@ -3303,6 +3303,7 @@ static void idr(H264Context *h){
         h->short_ref[i]= NULL;
     }
     h->short_ref_count=0;
+    h->prev_frame_num= 0;
 }
 
 /* forget old pics after a seek */