avformat/mov: Disallow empty sidx

It appears this is not allowed "Each Segment Index box documents how a (sub)segment is divided into one or more subsegments
(which may themselves be further subdivided using Segment Index boxes)."
Fixes: Null pointer dereference
Fixes: Ticket9517

Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavformat/mov.c

@@ -5118,6 +5118,8 @@ static int mov_read_sidx(MOVContext *c, AVIOContext *pb, MOVAtom atom)
     avio_rb16(pb); // reserved
 
     item_count = avio_rb16(pb);
+    if (item_count == 0)
+        return AVERROR_INVALIDDATA;
 
     for (i = 0; i < item_count; i++) {
         int index;