nat: Look at the assignment command and remember on which timeslot the data is

This information will be needed when we are trying to forward
MGCP connections to and from the BSC through the IPA protocol.
diff --git a/openbsc/src/nat/bsc_nat_utils.c b/openbsc/src/nat/bsc_nat_utils.c
index 3e44193..f1bb4b5 100644
--- a/openbsc/src/nat/bsc_nat_utils.c
+++ b/openbsc/src/nat/bsc_nat_utils.c
@@ -79,6 +79,7 @@
 	LOGP(DNAT, LOGL_DEBUG, "Destroy 0x%x <-> 0x%x mapping for con %p\n",
 	     sccp_src_ref_to_int(&conn->real_ref),
 	     sccp_src_ref_to_int(&conn->patched_ref), conn->bsc);
+	bsc_mgcp_clear(conn);
 	llist_del(&conn->list_entry);
 	talloc_free(conn);
 }