glusterd: Perform NULL check on rsp.op_errstr before using it

Change-Id: Id18b215a91cf016964ea98d2f414293b82167d24
BUG: 962362
Signed-off-by: Krutika Dhananjay <kdhananj@redhat.com>
Reviewed-on: http://review.gluster.org/4992
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>