Use explicit length check

Use OSMO_MIN macro to check for MSISDN length. This makes the code
cleaner and will, hopefully, aid static analysis tools.

Change-Id: Ic0fbeb8d248c74e54bfb51ba2cdea55c4f386ac7
Fixes: CID57879
diff --git a/src/gprs/sgsn_libgtp.c b/src/gprs/sgsn_libgtp.c
index 478d402..a8a1502 100644
--- a/src/gprs/sgsn_libgtp.c
+++ b/src/gprs/sgsn_libgtp.c
@@ -176,9 +176,7 @@
 
 	/* Put the MSISDN in case we have it */
 	if (mmctx->subscr && mmctx->subscr->sgsn_data->msisdn_len) {
-		pdp->msisdn.l = mmctx->subscr->sgsn_data->msisdn_len;
-		if (pdp->msisdn.l > sizeof(pdp->msisdn.v))
-			pdp->msisdn.l = sizeof(pdp->msisdn.v);
+		pdp->msisdn.l = OSMO_MIN(mmctx->subscr->sgsn_data->msisdn_len, sizeof(pdp->msisdn.v));
 		memcpy(pdp->msisdn.v, mmctx->subscr->sgsn_data->msisdn,
 			pdp->msisdn.l);
 	} else {