demux_null: mark as seekable

No reason not to, and enables some strange constructions with
--external-file (although the result is not too smooth for certain
reasons).

demux/demux_null.c

@@ -24,6 +24,7 @@ static int try_open_file(struct demuxer *demux, enum demux_check check)
     if (!bstr_startswith0(bstr0(demux->filename), "null://") &&
         check != DEMUX_CHECK_REQUEST)
         return -1;
+    demux->seekable = true;
     return 0;
 }