libmsc: fix memory leak (struct msgb) in msc_i_ran_enc()

Function msc_i_ran_enc() calls msc_role_ran_encode(), but unlike the
other callers of this function it does not free() the encoded message.

A simple solution would be to call msgb_free(), like it's done in
the other places.  But a more elegant solution is to modify function
msc_role_ran_encode(), so that it attaches the msgb to OTC_SELECT.
This way there is no need to call msgb_free() here and there.

This change fixes a memleak observed while running ttcn3-msc-test.

Change-Id: I741e082badc32ba9a97c1495c894e1d22e122e3a
Related: OS#5340
diff --git a/src/libmsc/msc_t.c b/src/libmsc/msc_t.c
index af0ddaa..43bc74e 100644
--- a/src/libmsc/msc_t.c
+++ b/src/libmsc/msc_t.c
@@ -145,7 +145,6 @@
 		return;
 
 	msub_role_dispatch(msc_t->c.msub, MSC_ROLE_A, MSC_A_EV_FROM_T_PREPARE_HANDOVER_FAILURE, &an_apdu);
-	msgb_free(an_apdu.msg);
 }
 
 static int msc_t_ho_request_decode_and_store_cb(struct osmo_fsm_inst *msc_t_fi, void *data,
@@ -238,7 +237,6 @@
 static int msc_t_send_stored_ho_request__decode_cb(struct osmo_fsm_inst *msc_t_fi, void *data,
 						  const struct ran_msg *ran_dec)
 {
-	int rc;
 	struct an_apdu an_apdu;
 	struct msc_t *msc_t = msc_t_priv(msc_t_fi);
 	struct osmo_sockaddr_str *rtp_ran_local = data;
@@ -263,9 +261,7 @@
 	};
 	if (!an_apdu.msg)
 		return -EIO;
-	rc = msc_t_down_l2_co(msc_t, &an_apdu, true);
-	msgb_free(an_apdu.msg);
-	return rc;
+	return msc_t_down_l2_co(msc_t, &an_apdu, true);
 }
 
 /* The MGW endpoint is created, we know our AoIP Transport Layer Address and can send the Handover Request to the RAN
@@ -472,9 +468,7 @@
 	if (!an_apdu.msg)
 		return -EIO;
 	/* Send to remote MSC via msc_a_remote role */
-	rc = msub_role_dispatch(msc_t->c.msub, MSC_ROLE_A, MSC_A_EV_FROM_T_PREPARE_HANDOVER_RESPONSE, &an_apdu);
-	msgb_free(an_apdu.msg);
-	return rc;
+	return msub_role_dispatch(msc_t->c.msub, MSC_ROLE_A, MSC_A_EV_FROM_T_PREPARE_HANDOVER_RESPONSE, &an_apdu);
 }
 
 static int msc_t_wait_ho_request_ack_decode_cb(struct osmo_fsm_inst *msc_t_fi, void *data,