performance/io-cache: check the inode context to be NULL before accessing

Change-Id: I475af7f8ffd5e5d8adbd2a74af20e56ad7751f69
BUG: 958108
Signed-off-by: Raghavendra Bhat <raghavendra@redhat.com>
Reviewed-on: http://review.gluster.org/4916
Reviewed-by: Amar Tumballi <amarts@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Reviewed-on: http://review.gluster.org/5077
Reviewed-by: Anand Avati <avati@redhat.com>