Fix issues found by coverity (ARMmbed#2087)

Fix remaining coverity issue:
-348373 Dereference after null check (dhcpv6_client_service.c)

Author: Arto Kinnunen

source/DHCPv6_client/dhcpv6_client_service.c

@@ -392,7 +392,9 @@ void dhcpv6_renew(protocol_interface_info_entry_t *interface, if_address_entry_t
     payload_len = libdhcpv6_address_request_message_len(srv_data_ptr->clientLinkIdType, srv_data_ptr->serverLinkType, 0, !dhcp_client.no_address_hint);
     payload_ptr = ns_dyn_mem_temporary_alloc(payload_len);
     if (payload_ptr == NULL) {
-        addr->state_timer = 200; //Retry after 20 seconds
+        if (addr) {
+            addr->state_timer = 200; //Retry after 20 seconds
+        }
         tr_error("Out of memory");
         return ;
     }