movdec: Avoid av_malloc(0) in stss

Found-by: Mateusz "j00ru" Jurczyk and Gynvael Coldwind
(cherry picked from commit 29a20ac4a1)

libavformat/mov.c

@@ -1503,6 +1503,8 @@ static int mov_read_stss(MOVContext *c, AVIOContext *pb, MOVAtom atom)
 
     av_dlog(c->fc, "keyframe_count = %d\n", entries);
 
+    if (!entries)
+        return 0;
     if (entries >= UINT_MAX / sizeof(int))
         return AVERROR_INVALIDDATA;
     sc->keyframes = av_malloc(entries * sizeof(int));