ext4: check for zero length extent explicitly

ext4: check for zero length extent explicitly

commit 2f974865ffdfe7b9f46a9940836c8b167342563d upstream.

The following commit introduced a bug when checking for zero length extent

5946d08 ext4: check for overlapping extents in ext4_valid_extent_entries()

Zero length extent could pass the check if lblock is zero.

Adding the explicit check for zero length back.

Signed-off-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>