e2fsck: abort if there is a corrupted directory block when rehashing

e2fsck: abort if there is a corrupted directory block when rehashing

In e2fsck pass 3a, when we are rehashing directories, at least in
theory, all of the directories should have had corruptions with
respect to directory entry structure fixed.  However, it's possible
(for example, if the user declined a fix) that we can reach this stage
of processing with a corrupted directory entries.

So check for that case and don't try to process a corrupted directory
block so we don't run into trouble in mutate_name() if there is a
zero-length file name.

Addresses: TALOS-2019-0973
Addresses: CVE-2019-5188
Signed-off-by: Theodore Ts'o <tytso@mit.edu>