mxfdec: Fix infinite loop in mxf_packet_timestamps()

This can happen if an index table segment has a very large IndexStartPosition.
zzuf3.mxf is an example of such a file.

libavformat/mxfdec.c

@@ -1712,7 +1712,7 @@ static int mxf_read_header(AVFormatContext *s, AVFormatParameters *ap)
  */
 static void mxf_packet_timestamps(MXFContext *mxf, AVPacket *pkt)
 {
-    int64_t next_ofs;
+    int64_t last_ofs = -1, next_ofs;
     MXFIndexTable *t = &mxf->index_tables[0];
 
     /* this is called from the OP1a demuxing logic, which means there may be no index tables */
@@ -1724,9 +1724,17 @@ static void mxf_packet_timestamps(MXFContext *mxf, AVPacket *pkt)
         if (mxf_edit_unit_absolute_offset(mxf, t, mxf->current_edit_unit + 1, NULL, &next_ofs, 0) < 0)
             break;
 
+        if (next_ofs <= last_ofs) {
+            /* large next_ofs didn't change or current_edit_unit wrapped around
+             * this fixes the infinite loop on zzuf3.mxf */
+            av_log(mxf->fc, AV_LOG_ERROR, "next_ofs didn't change. not deriving packet timestamps\n");
+            return;
+        }
+
         if (next_ofs > pkt->pos)
             break;
 
+        last_ofs = next_ofs;
         mxf->current_edit_unit++;
     }