config: fix several access(NULL) calls

config: fix several access(NULL) calls

When $HOME is unset, home_config_paths fails and returns NULL pointers
for user_config and xdg_config. Valgrind complains with Syscall param
access(pathname) points to unaddressable byte(s).

Don't call blindly access() on these variables, but test them for
NULL-ness before.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>