libmsc/gsm_09_11.c: fix: return trans from establish_nc_ss_trans()

It is expected that establish_nc_ss_trans() returns an allocated
transaction in successful case, or NULL in case of error. The
function assumes two scenarios:

  - the subscriber already has an active RAN connection,
  - RAN connection needs to be established (Paging).

In the first case, a pointer to the transaction is returned as
expected, but in case of Paging, NULL has always been returned,
even if there were no errors. Let's fix this.

Change-Id: I9dcee64dd0b435ef29630c223132b81724701f93
diff --git a/src/libmsc/gsm_09_11.c b/src/libmsc/gsm_09_11.c
index 3de933c..39b9711 100644
--- a/src/libmsc/gsm_09_11.c
+++ b/src/libmsc/gsm_09_11.c
@@ -409,7 +409,7 @@
 	msgb_tlv_put(trans->ss.msg, GSM0480_IE_FACILITY,
 		gsup_msg->ss_info_len, gsup_msg->ss_info);
 
-	return NULL;
+	return trans;
 }
 
 /* NC SS specific transaction release.