smpp_openbsc.c: check acl before deref it

All other calls check acl before deref because in a setup
with no access policy, there won't be any acl structure

Change-Id: Ibe0256535b40351594d79baa05a0147a9f89dc26
diff --git a/src/libmsc/smpp_openbsc.c b/src/libmsc/smpp_openbsc.c
index bbfc500..e4c3891 100644
--- a/src/libmsc/smpp_openbsc.c
+++ b/src/libmsc/smpp_openbsc.c
@@ -326,7 +326,7 @@
 				"ESME is not (yet) bound, skipping alert\n");
 			continue;
 		}
-		if (!esme->acl->alert_notifications) {
+		if (esme->acl && !esme->acl->alert_notifications) {
 			LOGP(DSMPP, LOGL_DEBUG,
 				"[%s] is not set to receive Alert Notifications\n",
 					esme->system_id);