tty, serial: Fix race and NULL check in uart_close()

Commit 46d57a449aa1 ("serial: use tty_port pointers in the core code")
contained two bugs that causes (rare) crashes:

- the rename typoed one site

- a NULL check was missed

Reported-and-tested-by: Ingo Molnar <[e-mail]>
Cc: Greg KH <[e-mail]>
Cc: Alan Cox <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/serial/serial_core.c

···
 
 	BUG_ON(!kernel_locked());
 
+	if (!state)
+		return;
+
 	uport = state->uart_port;
 	port = &state->port;
 
···
 	 */
 	if (port->flags & ASYNC_INITIALIZED) {
 		unsigned long flags;
-		spin_lock_irqsave(&port->lock, flags);
+		spin_lock_irqsave(&uport->lock, flags);
 		uport->ops->stop_rx(uport);
-		spin_unlock_irqrestore(&port->lock, flags);
+		spin_unlock_irqrestore(&uport->lock, flags);
 		/*
 		 * Before we drop DTR, make sure the UART transmitter
 		 * has completely drained; this is especially