[NETFILTER]: nf_conntrack_sip: check sname != NULL before calling strncmp

The check got lost during the conversion to nf_conntrack.
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/netfilter/nf_conntrack_sip.c

@@ -330,7 +330,8 @@ int ct_sip_get_info(struct nf_conn *ct,
 
 	while (dptr <= limit) {
 		if ((strncmp(dptr, hnfo->lname, hnfo->lnlen) != 0) &&
-		    (strncmp(dptr, hnfo->sname, hnfo->snlen) != 0)) {
+		    (hnfo->sname == NULL ||
+		     strncmp(dptr, hnfo->sname, hnfo->snlen) != 0)) {
 			dptr++;
 			continue;
 		}