libosmo-mgcp: do not use the default msgb talloc context

Trunk is safe, since it will not disappear sooner than the endpoints or
connections.

osmux still missing!

Change-Id: I15b01085f31e9a10a1ad381713ca2275356ca20c
diff --git a/src/libosmo-mgcp/mgcp_network.c b/src/libosmo-mgcp/mgcp_network.c
index cb82137..86b0d06 100644
--- a/src/libosmo-mgcp/mgcp_network.c
+++ b/src/libosmo-mgcp/mgcp_network.c
@@ -1448,13 +1448,14 @@
 	int ret;
 	enum rtp_proto proto;
 	struct osmo_rtp_msg_ctx *mc;
-	struct msgb *msg = msgb_alloc(RTP_BUF_SIZE, "RTP-rx");
+	struct msgb *msg;
 	int rc;
 
 	conn_src = (struct mgcp_conn_rtp *)fd->data;
 	OSMO_ASSERT(conn_src);
 	endp = conn_src->conn->endp;
 	OSMO_ASSERT(endp);
+	msg = msgb_alloc_c(endp->trunk, RTP_BUF_SIZE, "RTP-rx");
 
 	proto = (fd == &conn_src->end.rtp)? MGCP_PROTO_RTP : MGCP_PROTO_RTCP;