gdesktopappinfo: Validate XDG_CURRENT_DESKTOP before using it

Its components are used to build filenames, so if the value of
`XDG_CURRENT_DESKTOP` comes from an untrusted caller (as can happen in
setuid programs), using it unvalidated may be unsafe.

Signed-off-by: Philip Withnall <pwithnall@endlessos.org>

Helps: #2168