ext4: fix coverity warning on error path of filename setup

Fix the following coverity warning reported by Dan Carpenter:

fs/ext4/namei.c:1311 ext4_fname_setup_ci_filename()
	  warn: 'cf_name->len' unsigned <= 0

Fixes: 3ae72562

 ("ext4: optimize case-insensitive lookups")
Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>