RPC error message size fixed.

In case of RPC error, special error message passed to handler.
Field 'size' expected to be 0 in this case because in contains fake
empty buffer.

src/nxt_port_rpc.c

@@ -489,7 +489,7 @@ nxt_port_rpc_close(nxt_task_t *task, nxt_port_t *port)
         msg.port_msg.nf = 0;
         msg.port_msg.mf = 0;
         msg.port_msg.tracking = 0;
-        msg.size = sizeof(msg.port_msg);
+        msg.size = 0;
         msg.cancelled = 0;
         msg.u.data = NULL;