xlator: NULL terminate volume_options struct

Problem:
volume_options struct for open-behind and quick-read xlators
were not NULL terminated.

Fix:
Make them NULL terminated.

Change-Id: I2615a1f15c6e5674030a219a99ddf91596bf346b
BUG: 965995
Signed-off-by: Santosh Kumar Pradhan <spradhan@redhat.com>
Reviewed-on: http://review.gluster.org/5064
Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>