rdp: Don't dereference NULL pointer when parent dir cannot be found

Malicious remote desktop clients can send invalid file lists containing
illegal filenames, such as filenames containing the '/' character.
In such case, gnome-remote-desktop currently dereferences a NULL
pointer, which would lead to a crash.

While such malicious remote desktop clients are not known, still add
handling for such situation by discarding the file list, when such
situation is detected.