Require 4 instead of 3 frames for detecting mp3,

fixes misdetection of mpegps_mp3_unrecognized_format.mpg (issue997)

Originally committed as revision 18657 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/mp3.c

@@ -391,10 +391,11 @@ static int mp3_read_probe(AVProbeData *p)
     }
     if   (first_frames>=3) return AVPROBE_SCORE_MAX/2+1;
     else if(max_frames>500)return AVPROBE_SCORE_MAX/2;
-    else if(max_frames>=3) return AVPROBE_SCORE_MAX/4;
+    else if(max_frames>=4) return AVPROBE_SCORE_MAX/4;
     else if(buf0!=p->buf)  return AVPROBE_SCORE_MAX/4-1;
     else if(max_frames>=1) return 1;
     else                   return 0;
+//mpegps_mp3_unrecognized_format.mpg has max_frames=3
 }
 
 /**