avformat/mvdec: Use 64bit for ret to avoid overflow

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit 26c0cc15)
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mvdec.c

@@ -371,7 +371,7 @@ static int mv_read_packet(AVFormatContext *avctx, AVPacket *pkt)
     AVStream *st = avctx->streams[mv->stream_index];
     const AVIndexEntry *index;
     int frame = mv->frame[mv->stream_index];
-    int ret;
+    int64_t ret;
     uint64_t pos;
 
     if (frame  < st->nb_index_entries) {