e2fsprogs: allow 0-length xattr values in e2fsck

e2fsprogs: allow 0-length xattr values in e2fsck

e2fsck thinks that this:

# touch mnt/testfile1
# setfattr -n "user.test" mnt/testfile1

results in a filesystem with corruption:

Pass 1: Checking inodes, blocks, and sizes
Extended attribute in inode 12 has a value size (0) which is invalid
Clear? yes

but as far as I can tell, there is absolutely nothing wrong with
a 0-length value on an extended attribute.  Just remove the check.

Reported-by: David Shaw <dshaw@jabberwocky.com>
Reported-by: Harald Reindl <h.reindl@thelounge.net>
Addresses-Red-Hat-Bugzilla: #557959
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>