Remove BUG_ON from n_tty_read()

Change the BUG_ON to WARN_ON and return in case of tty->read_buf==NULL. We want to track a
couple of long standing reports of this but at the same time we can avoid killing the box.

Signed-off-by: Stanislav Kozina <[e-mail]>
Signed-off-by: Alan Cox <[e-mail]>
Cc: Horses <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

drivers/tty/n_tty.c

···
 
 do_it_again:
 
-	BUG_ON(!tty->read_buf);
+	if (WARN_ON(!tty->read_buf))
+		return -EAGAIN;
 
 	c = job_control(tty, file);
 	if (c < 0)