dm-crypt fix for zero-length key.
Signed-Off-By: Christophe Saout <christophe@saout.de>
Signed-Off-By: Alasdair G Kergon <agk@redhat.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>