storage/posix: skip path construction when dentry list is empty

This is a minor latency optimization to the readdirp path in
storage/posix. During a recursive list, we hit this codepath with
an empty list once per high-level directory to read when end of
directory is reached. Skip constructing hpath, since we don't do
anything with it in this case.

BUG: 903175
Change-Id: I98d7c65505205d55575f064b1e982700f1320cc0
Signed-off-by: Brian Foster <bfoster@redhat.com>
Reviewed-on: http://review.gluster.org/4432
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>