files: handle systems that disallow NULL as first parameter of getcwd

(This change will be made superfluous when we start using gnulib.)

This prevents getcwd() from failing on Android and thus completes the
fix for https://savannah.gnu.org/bugs/index.php?47659.

Reported-by: Chris Renshaw <osm0sis@outlook.com>
Signed-off-by: Benno Schulenberg <bensberg@justemail.net>
1 file changed