prevent O(n^2) execution of ff_mpa_decode_header() for valid mp3 files

Originally committed as revision 11144 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/mp3.c

@@ -418,6 +418,8 @@ static int mp3_read_probe(AVProbeData *p)
         max_frames = FFMAX(max_frames, frames);
         if(buf == p->buf)
             first_frames= frames;
+        if(buf2 > end)
+            break;
     }
     if   (first_frames>=3) return AVPROBE_SCORE_MAX/2+1;
     else if(max_frames>=3) return AVPROBE_SCORE_MAX/4;