nfs: prevent potential NULL dereference in mnt3_readlink_cbk()

In case 'mres' is NULL, we jump to 'mnterr' where 'mres->req' will be
used. This will cause a segmentation fault, which we really do not want.

Change-Id: I01d6f3aa0343def54dcd4a5610001ff7d62c8834
CID: 1256179
BUG: 789278
Signed-off-by: Niels de Vos <ndevos@redhat.com>
Reviewed-on: http://review.gluster.org/9257
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: jiffin tony Thottan <jthottan@redhat.com>
Reviewed-by: Humble Devassy Chirammal <humble.devassy@gmail.com>