vcsim: avoid possible panic in UnregisterVM_Task
vcsim: avoid possible panic in UnregisterVM_Task

It is possible the VM's parent folder is destroyed in the middle of a VM Unregister call,
avoid a panic in that case.  See also: 1427d581 (#1744)

bruceadowns/govmomi,vmware/govmomi,vmware/govmomi,bruceadowns/govmomi,bruceadowns/govmomi,vmware/govmomi