avformat/mov: Handle when we have an mfra box but have not read the full sidx for a fragment

Use the tfra timestamp if it is available and sidx timestamp is not.

Fixes reading the entire file after seeking in a live-style DASH FMP4
with an MFRA.

This specifically fixes when use_mfra_for is set.

Signed-off-by: Derek Buitenhuis <derek.buitenhuis@gmail.com>

libavformat/mov.c

@@ -1279,6 +1279,10 @@ static int64_t get_frag_time(MOVFragmentIndex *frag_index,
 
     if (track_id >= 0) {
         frag_stream_info = get_frag_stream_info(frag_index, index, track_id);
+        if (frag_stream_info->sidx_pts != AV_NOPTS_VALUE)
+            return frag_stream_info->sidx_pts;
+        if (frag_stream_info->first_tfra_pts != AV_NOPTS_VALUE)
+            return frag_stream_info->first_tfra_pts;
         return frag_stream_info->sidx_pts;
     }