Several callers access the length output argument, expecting
it to be zero also on failure. That is a bug, because glib does
not guarantee that.

Fix that by making a stronger promise from our wrappers: the output
argument should also be set on failure.

Also ensure that calls to g_keyfile_get_groups() and
g_keyfile_get_keys() don't rely on the length output to be valid,
when the function call fails.

Actually, these issues were not severe because:

- `g_key_file_get_*_list()`'s implementation always sets the output length
  even on failure (undocumented).
- `g_key_file_get_groups()` cannot fail and always set the length.
- `g_key_file_get_keys()` is called under circumstances where it won't
  fail.

Still, be explicit about it.