mgmt/glusterd: check brickinfo for NULL before dereferencing it

Change-Id: I59fed6adf13d6d72d79c986bf3578ca79b9756ac
BUG: 781953
Signed-off-by: Raghavendra Bhat <raghavendrabhat@gluster.com>
Reviewed-on: http://review.gluster.org/2648
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>