Cluster/DHT : Fixed crash due to null deref

A lookup on a linkto file whose trusted.glusterfs.dht.linkto
xattr points to a subvol that is not part of the volume
can cause the brick process to segfault due to a null dereference.
Modified to check for a non-null value before attempting to access
the variable.

Change-Id: Ie8f9df058f842cfc0c2b52a8f147e557677386fa
BUG: 1159571
Signed-off-by: Nithya Balachandran <nbalacha@redhat.com>
Reviewed-on: http://review.gluster.org/9034
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: venkatesh somyajulu <vsomyaju@redhat.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>