Fix remotely exploitable arbitrary code execution vulnerability.

Found by Tobias Klein / tk // trapkit / de /
See: http://www.trapkit.de/advisories/TKADV2009-004.txt

Originally committed as revision 16846 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/4xm.c

@@ -166,12 +166,13 @@ static int fourxm_read_header(AVFormatContext *s,
                 goto fail;
             }
             current_track = AV_RL32(&header[i + 8]);
-            if (current_track + 1 > fourxm->track_count) {
-                fourxm->track_count = current_track + 1;
-                if((unsigned)fourxm->track_count >= UINT_MAX / sizeof(AudioTrack)){
+            if((unsigned)current_track >= UINT_MAX / sizeof(AudioTrack) - 1){
+                av_log(s, AV_LOG_ERROR, "current_track too large\n");
                 ret= -1;
                 goto fail;
             }
+            if (current_track + 1 > fourxm->track_count) {
+                fourxm->track_count = current_track + 1;
                 fourxm->tracks = av_realloc(fourxm->tracks,
                     fourxm->track_count * sizeof(AudioTrack));
                 if (!fourxm->tracks) {