Breaking read loop by nxt_port_read_close().

Port message handler may perform fork() and then close port read file
descriptor and enable write on same event fd.  Next read attempt in this case
may cause different errors in log file.

src/nxt_port_socket.c

@@ -461,6 +461,7 @@ nxt_port_read_enable(nxt_task_t *task, nxt_port_t *port)
 void
 nxt_port_read_close(nxt_port_t *port)
 {
+    port->socket.read_ready = 0;
     nxt_socket_close(port->socket.task, port->pair[0]);
     port->pair[0] = -1;
 }