Print warning if scale/rate is invalid.

Originally committed as revision 12806 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/avidec.c

@@ -380,6 +380,7 @@ static int avi_read_header(AVFormatContext *s, AVFormatParameters *ap)
             ast->scale = get_le32(pb);
             ast->rate = get_le32(pb);
             if(!(ast->scale && ast->rate)){
+                av_log(s, AV_LOG_WARNING, "Scale/Rate is %u/%u which is invalid. (This file has been generated by broken software)\n", ast->scale, ast->rate);
                 if(frame_period){
                     ast->rate = 1000000;
                     ast->scale = frame_period;