ipvs: ip_vs_sh: ip_vs_sh_get_port: check skb_header_pointer for NULL

skb_header_pointer could return NULL, so check for it as we do it
everywhere else in ipvs code. This fixes a coverity warning.
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Acked-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: Simon Horman <horms@verge.net.au>

net/netfilter/ipvs/ip_vs_sh.c

@@ -269,14 +269,20 @@ ip_vs_sh_get_port(const struct sk_buff *skb, struct ip_vs_iphdr *iph)
 	switch (iph->protocol) {
 	case IPPROTO_TCP:
 		th = skb_header_pointer(skb, iph->len, sizeof(_tcph), &_tcph);
+		if (unlikely(th == NULL))
+			return 0;
 		port = th->source;
 		break;
 	case IPPROTO_UDP:
 		uh = skb_header_pointer(skb, iph->len, sizeof(_udph), &_udph);
+		if (unlikely(uh == NULL))
+			return 0;
 		port = uh->source;
 		break;
 	case IPPROTO_SCTP:
 		sh = skb_header_pointer(skb, iph->len, sizeof(_sctph), &_sctph);
+		if (unlikely(sh == NULL))
+			return 0;
 		port = sh->source;
 		break;
 	default: