[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 <benh@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>