nfs/server: Fix multiple crashes in acl handling code.

Change-Id: I9b39a485c8b98d9eabe6153487f4dfbd26f8af13
BUG: 915280
Signed-off-by: Vijay Bellur <vbellur@redhat.com>
Reviewed-on: http://review.gluster.org/4578
Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Anand Avati <avati@redhat.com>