UBIFS: add a couple of extra asserts

... to catch possible memory corruptions.

Change-Id: I216165e8b8c897c228e88d44ccfb9a343fbf204a
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Git-commit: fb4325a3d9f983160f142b919880ccbe2304bc25
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Nikhilesh Reddy <reddyn@codeaurora.org>