[PATCH] md/bitmap.c:bitmap_mask_state(): fix inconsequent NULL checking

We dereference bitmap both one line above and one line below this check
rendering this check quite useless.

Spotted by the Coverity checker.

Signed-off-by: Adrian Bunk <bunk@stusta.de>
Cc: Alasdair G Kergon <agk@redhat.com>
Cc: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>