* prevent crash in decoder

Originally committed as revision 179 to svn://svn.ffmpeg.org/ffmpeg/trunk
diff --git a/libavcodec/msmpeg4.c b/libavcodec/msmpeg4.c
index aa42029..01e3d54 100644
--- a/libavcodec/msmpeg4.c
+++ b/libavcodec/msmpeg4.c
@@ -664,7 +664,7 @@
 
     /* special slice handling */
     if (s->mb_x == 0) {
-        if ((s->mb_y % s->slice_height) == 0) {
+        if (s->slice_height && (s->mb_y % s->slice_height) == 0) {
             int wrap;
             /* reset DC pred (set previous line to 1024) */
             wrap = 2 * s->mb_width + 2;