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

Change-Id: I2ae30f08965b26a21db541f87de78772cb17135a
BUG: 962362
Signed-off-by: Krutika Dhananjay <kdhananj@redhat.com>
Reviewed-on: http://review.gluster.org/4996
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>