Check 4xm headers for validity. This should avoid 1/0.

Originally committed as revision 26392 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/4xm.c

@@ -185,6 +185,13 @@ static int fourxm_read_header(AVFormatContext *s,
             fourxm->tracks[current_track].sample_rate = AV_RL32(&header[i + 40]);
             fourxm->tracks[current_track].bits        = AV_RL32(&header[i + 44]);
             fourxm->tracks[current_track].audio_pts   = 0;
+            if(   fourxm->tracks[current_track].channels    <= 0
+               || fourxm->tracks[current_track].sample_rate <= 0
+               || fourxm->tracks[current_track].bits        <  0){
+                av_log(s, AV_LOG_ERROR, "audio header invalid\n");
+                ret= -1;
+                goto fail;
+            }
             i += 8 + size;
 
             /* allocate a new AVStream */
@@ -291,7 +298,7 @@ static int fourxm_read_packet(AVFormatContext *s,
             out_size= get_le32(pb);
             size-=8;
 
-            if (track_number < fourxm->track_count) {
+            if (track_number < fourxm->track_count && fourxm->tracks[track_number].channels>0) {
                 ret= av_get_packet(s->pb, pkt, size);
                 if(ret<0)
                     return AVERROR(EIO);