MT#61856 media_socket: add guard for child media extmap

Just add a guard, and don't assume by default that
the child media has a mid extmap entry.

Change-Id: I6fd7bef63b521d0cff7866ca54bb8e49ec893cec

daemon/media_socket.c

@@ -2869,6 +2869,8 @@ static void rtp_ext_mid_kernel(kernelize_state *s, unsigned int component, struc
 		return;
 	if (!bundle->extmap_id[RTP_EXT_MID])
 		return;
+	if (!media->extmap_id[RTP_EXT_MID])
+		return;
 	if (bundle->extmap_id[RTP_EXT_MID]->id != media->extmap_id[RTP_EXT_MID]->id)
 		return;