Fix a memory leak in grpc_udp_server_add_port()

This occurred when running a QUIC test in an IPv6-only environment:

E1123 16:04:20.282041  159850 heap-profile-table.cc:489] RAW: Leak of 1360 bytes in 17 objects allocated from:
        @ 0x7f160b8923ab gpr_malloc
        @ 0x7f160b8cff27 copy_error_and_unref
        @ 0x7f160b8d0151 grpc_error_set_str
        @ 0x7f160b8e3ff8 error_for_fd
        @ 0x7f160b8e3edd grpc_create_dualstack_socket
        @ 0x7f160b8ea5c3 grpc_udp_server_add_port
        @ 0x7f1627a65d20 grpc_server_add_quic_port
        @ 0x7f1627a6bea4 grpc::(anonymous namespace)::QuicServerCredentialsImpl::AddPortToServer()
        ...

src/core/lib/iomgr/udp_server.c

@@ -388,7 +388,9 @@ int grpc_udp_server_add_port(grpc_udp_server *s,
     /* Try listening on IPv6 first. */
     addr = &wild6;
     // TODO(rjshade): Test and propagate the returned grpc_error*:
-    grpc_create_dualstack_socket(addr, SOCK_DGRAM, IPPROTO_UDP, &dsmode, &fd);
+    GRPC_ERROR_UNREF(
+        grpc_create_dualstack_socket(addr, SOCK_DGRAM, IPPROTO_UDP,
+                                     &dsmode, &fd));
     allocated_port1 = add_socket_to_server(s, fd, addr, read_cb, orphan_cb);
     if (fd >= 0 && dsmode == GRPC_DSMODE_DUALSTACK) {
       goto done;
@@ -402,7 +404,9 @@ int grpc_udp_server_add_port(grpc_udp_server *s,
   }
 
   // TODO(rjshade): Test and propagate the returned grpc_error*:
-  grpc_create_dualstack_socket(addr, SOCK_DGRAM, IPPROTO_UDP, &dsmode, &fd);
+  GRPC_ERROR_UNREF(
+      grpc_create_dualstack_socket(addr, SOCK_DGRAM, IPPROTO_UDP,
+                                   &dsmode, &fd));
   if (fd < 0) {
     gpr_log(GPR_ERROR, "Unable to create socket: %s", strerror(errno));
   }