sgsn: Fix pattern for too long msisdn

We don't care how many bytes the length has but the
destination we want to copy to.

Fixes: CID#1296813
diff --git a/openbsc/src/gprs/sgsn_libgtp.c b/openbsc/src/gprs/sgsn_libgtp.c
index 6aa12b2..aff2f91 100644
--- a/openbsc/src/gprs/sgsn_libgtp.c
+++ b/openbsc/src/gprs/sgsn_libgtp.c
@@ -162,7 +162,7 @@
 	if (mmctx->subscr) {
 		pdp->msisdn.l = mmctx->subscr->sgsn_data->msisdn_len;
 		if (pdp->msisdn.l > sizeof(pdp->msisdn.v))
-			pdp->msisdn.l = sizeof(pdp->msisdn.l);
+			pdp->msisdn.l = sizeof(pdp->msisdn.v);
 		memcpy(pdp->msisdn.v, mmctx->subscr->sgsn_data->msisdn,
 			pdp->msisdn.l);
 	}