gprs_ns2: ensure the incoming NSVC is also the outgoing NSVC

Previous the SNS NSVC (the NSVC used for all SNS traffic) was never changed except
when the choosen NSVC went dead or got freed.
When receiving a SNS SIZE PDU over a different NSVC than the current SNS
NSVC the answer would be transmitted to a different port.

Change-Id: I36cd9488b8bca5cb99dae5cf50a55ee282e0557b
diff --git a/src/gb/gprs_ns2_sns.c b/src/gb/gprs_ns2_sns.c
index c1be0ab..7619b66 100644
--- a/src/gb/gprs_ns2_sns.c
+++ b/src/gb/gprs_ns2_sns.c
@@ -2326,8 +2326,7 @@
 	/* FIXME: how to resolve SNS FSM Instance by NSEI (SGSN)? */
 	fi = nse->bss_sns_fi;
 	gss = (struct ns2_sns_state *) fi->priv;
-	if (!gss->sns_nsvc)
-		gss->sns_nsvc = nsvc;
+	gss->sns_nsvc = nsvc;
 
 	LOGPFSML(fi, LOGL_DEBUG, "NSEI=%u Rx SNS PDU type %s\n", nsei,
 		 get_value_string(gprs_ns_pdu_strings, nsh->pdu_type));