folder_branch_ops: fix crash during partial sync mark-and-sweep
folder_branch_ops: fix crash during partial sync mark-and-sweep

If the synced paths contain a symlink, it would crash because
`childNode` would be `nil`.

keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client,keybase/client