Don't forget to initialize `int error' in ttydev_open().

I've had some reports in the past that opening an already opened TTY
through, for example, /dev/tty can fail with random error codes. Looking
at ttydev_open(), I can see there is a way `error' is returned without
initialising it. Even though I haven't had any confirmation this fixes
the bug, I'll fix it anyway.

Reported by:	Andrzej Tobola <ato iem pw edu pl>

sys/kern/tty.c

@@ -204,7 +204,7 @@ static int
 ttydev_open(struct cdev *dev, int oflags, int devtype, struct thread *td)
 {
 	struct tty *tp = dev->si_drv1;
-	int error;
+	int error = 0;
 
 	/* Disallow access when the TTY belongs to a different prison. */
 	if (dev->si_cred != NULL &&