performance/quick-read: Check for iobuf and iobref before unref.

In the absence of this check, iobuf_unref() can cause a log flood
for read operations.

Change-Id: Ib8beef6e55b69ecfb566bf61a96215ef0d0d74dc
BUG: 1116514
Signed-off-by: Vijay Bellur <vbellur@redhat.com>
Reviewed-on: http://review.gluster.org/8242
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Humble Devassy Chirammal <humble.devassy@gmail.com>
Reviewed-by: Prashanth Pai <ppai@redhat.com>
Reviewed-by: Kaleb KEITHLEY <kkeithle@redhat.com>