Fix crash in dht_migration_complete_check_task because of NULL fd

This is a backport of Ia5a5d40bcea7bfb320ef7096af1e035b8847d4ff

BUG: 960055
Change-Id: Ibf3547a775d7ca2f3a097c880cdf38ffafb322da
Signed-off-by: Emmanuel Dreyfus <manu@netbsd.org>
Reviewed-on: http://review.gluster.org/5139
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Jeff Darcy <jdarcy@redhat.com>