use calloc for new_interface_server

malloc left garbage in the structure, dns IPs were filled with random content

Author: JojoS62

connectivity/lwipstack/lwip/src/core/lwip_dns.c

@@ -431,7 +431,7 @@ dns_add_interface_server(u8_t numdns, const char *interface_name, const ip_addr_
     }
   }
   // add new dns server to the list tail
-  new_interface_server = mem_malloc(sizeof(struct dns_server_interface));
+  new_interface_server = mem_calloc(1, sizeof(struct dns_server_interface));
   snprintf(new_interface_server->interface_name, INTERFACE_NAME_MAX_SIZE, "%s",interface_name);
   new_interface_server->dns_servers[numdns] = (*dnsserver);
   new_interface_server->next = NULL;