system/posix-acl: prevent NULL pointer dereference of group_ce

Thanks Amar Tumballi.

Change-Id: I3ac9b46d4c3fcd12d1eec779317a03c47d267556
BUG: 887098
Signed-off-by: Varun Shastry <vshastry@redhat.com>
Reviewed-on: http://review.gluster.org/4395
Reviewed-by: Anand Avati <avati@redhat.com>
Tested-by: Anand Avati <avati@redhat.com>