cli: check for null in is_server_debug_xlator()

Command: gluster volume set <volname> diagnostics.client-log-level trace
Expected output:
"volume set: failed: option log-level trace: 'trace' is not valid
(possible options are DEBUG, WARNING, ERROR, INFO, CRITICAL, NONE,
TRACE.)"
Current output: gluster cli receives a segmentation fault
Fix: check for NULL before calling strstr

Change-Id: If4c7a85a635849a388cf122543e12349c109643c
BUG: 982174
Signed-off-by: Ravishankar N <ravishankar@redhat.com>
Reviewed-on: http://review.gluster.org/5298
Reviewed-by: Kaushal M <kaushal@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>