Allow using only the mfra info for seeking using the fragment index

The mfra has enough information to enable seeking, and reading it is
behind an AVOption flag, so we shouldn't require that sidx information
also be present in order to seek using the fragment index.

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

libavformat/mov.c

@@ -7529,6 +7529,7 @@ static int mov_read_mfra(MOVContext *c, AVIOContext *f)
             goto fail;
     } while (!ret);
     ret = 0;
+    c->frag_index.complete = 1;
 fail:
     seek_ret = avio_seek(f, original_pos, SEEK_SET);
     if (seek_ret < 0) {