Besides freeing initrd memory, also clear out the now dangling pointers to
it, to make sure accidental late use attempts can be detected.

Signed-off-by: Jan Beulich <jbeulich@novell.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>