FTP backend Exists() checks if wrong entry is directory

[funkyshu]

Describe the bug
Calling Exists sometimes attempts to create a directory that already exists because we're checking for entries[0] rather the the current entry at https://github.com/C2FO/vfs/blob/v6.14.0/backend/ftp/location.go#L160

To Reproduce
Create a file and subdirectory in some base dir, then call Exists on the subdirectory. If the file entry is returned first the entry type check will always be "file".

Expected behavior
Exists should properly check for the entry type of the current entry.