4xm: check that bits per sample is strictly positive

Avoids a divide by zero.

Reported-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
CC: libav-stable@libav.org
(cherry picked from commit a7c1689ded)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

libavformat/4xm.c

@@ -154,7 +154,7 @@ static int parse_strk(AVFormatContext *s,
 
     if (fourxm->tracks[track].channels    <= 0 ||
         fourxm->tracks[track].sample_rate <= 0 ||
-        fourxm->tracks[track].bits        < 0) {
+        fourxm->tracks[track].bits        <= 0) {
         av_log(s, AV_LOG_ERROR, "audio header invalid\n");
         return AVERROR_INVALIDDATA;
     }