[PATCH] Fix crash in unregister_console()

If unregister_console() is inadvertently called while no consoles are
registered, it will crash trying to dereference NULL pointer. It is
necessary to fix that because register_console() provides no indication
that it actually registered the console passed in. In fact, it may well
decide not to register it based on various things...

(akpm: It'd be better to make register_console() return something and fix the
callers. All 106 of them...)

Signed-off-by: Benjamin Herrenschmidt <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

kernel/printk.c

···
 	if (console_drivers == console) {
 		console_drivers=console->next;
 		res = 0;
-	} else {
+	} else if (console_drivers) {
 		for (a=console_drivers->next, b=console_drivers ;
 		     a; b=a, a=b->next) {
 			if (a == console) {