avformat/mov: force parsing of headers if stts is absent

Fixes Ticket2991
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/mov.c

@@ -1790,6 +1790,8 @@ static int mov_read_stss(MOVContext *c, AVIOContext *pb, MOVAtom atom)
     if (!entries)
     {
         sc->keyframe_absent = 1;
+        if (!st->need_parsing)
+            st->need_parsing = AVSTREAM_PARSE_HEADERS;
         return 0;
     }
     if (entries >= UINT_MAX / sizeof(int))