test_sockets: fix warning about unused variable "addr_size"

test/unit/api/test_sockets.c

@@ -208,6 +208,7 @@ static void test_sockets_sendmsg_udp(int domain)
       break;
 #endif /* LWIP_IPV4 */
     default:
+      addr_size = 0;
       fail();
       break;
   }