4xm: Check that the read track value is non-negative

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
Signed-off-by: Martin Storsjö <martin@martin.st>

libavformat/4xm.c

@@ -131,6 +131,8 @@ static int parse_strk(AVFormatContext *s,
         return AVERROR_INVALIDDATA;
 
     track = AV_RL32(buf + 8);
+    if (track < 0)
+        return AVERROR_INVALIDDATA;
     if (track + 1 > fourxm->track_count) {
         if (av_reallocp_array(&fourxm->tracks, track + 1, sizeof(AudioTrack)))
             return AVERROR(ENOMEM);