gfapi: handle failures in graph switches

A graph switch failure can potentially result in NULL returned
for glfs_active_subvol(). Handle this gracefully in glfs_subvol_done()

Change-Id: I6adbd878c354b9ed2314fc71cea4671792efa0a4
BUG: 953694
Signed-off-by: Anand Avati <avati@redhat.com>
Reviewed-on: http://review.gluster.org/5427
Reviewed-by: Vijay Bellur <vbellur@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>