fuse(efence): zero sized memory was being allocated.

For the last call of getdents(), gf_malloc called with 0 size, which is then
caught by efence.

BUG: 782760
Change-Id: If289029117a62ecfcecc70480e5ac8f0e050487d
Signed-off-by: Anand Avati <avati@redhat.com>
Original-author: Varun Shastry <vshastry@redhat.com>
Reviewed-on: http://review.gluster.org/3846
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Amar Tumballi <amarts@redhat.com>