use g_debug(format, ...) safely

* pinentry/password-cache.c (password_cache_clear): use g_debug safely
  in case error->message is malformed.

Without this change, with -Werror=format-security, we see:

password-cache.c: In function ‘password_cache_clear’:
password-cache.c:153:7: error: format not a string literal and no format arguments [-Werror=format-security]

g_debug(error->message);
^