uml: line.c: avoid NULL pointer dereference

Assign tty only if line is not NULL.

[akpm@linux-foundation.org: simplification]
Signed-off-by: Alexander Beregalov <a.beregalov@gmail.com>
Cc: Jeff Dike <jdike@addtoit.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

arch/um/drivers/line.c

@@ -18,10 +18,10 @@ static irqreturn_t line_interrupt(int irq, void *data)
 {
 	struct chan *chan = data;
 	struct line *line = chan->line;
-	struct tty_struct *tty = line->tty;
+	struct tty_struct *tty;
 
 	if (line)
-		chan_interrupt(&line->chan_list, &line->task, tty, irq);
+		chan_interrupt(&line->chan_list, &line->task, line->tty, irq);
 	return IRQ_HANDLED;
 }