Squashfs: fix corruption check in get_dir_index_using_name()

Patch "Squashfs: sanity check information from disk" from
Dan Carpenter adds a missing check for corruption in the
"size" field while reading the directory index from disk.

It, however, sets err to -EINVAL, this value is not used later, and
so setting it is completely redundant.  So remove it.

Errors in reading the index are deliberately non-fatal.  If we
get an error in reading the index we just return the part of the
index we have managed to read - the index isn't essential,
just quicker.

Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>