Will Thompson authored Sep 20, 2018 and Philip Withnall committed Oct 01, 2018

Previously, in the case where 'kf' has more than one ref, calling
g_key_file_free(kf) would break it. For example, calling
g_key_file_has_key(kf, ...) would hit the following assertion:

    g_hash_table_lookup: assertion 'hash_table != NULL' failed

This is because g_key_file_free() calls g_key_file_clear() which sets
self->groups and other fields to NULL; most lookup functions assume
these fields are non-NULL.

One fix would be to call g_key_file_init() right after
g_key_file_clear() in g_key_file_free(). However, in the case where
there are no other refs to the keyfile, this would mean allocating
many new hash tables which will be immediately destroyed when
g_key_file_unref() removes the last ref. Instead, inline the unref, and
re-initialize the internal state when the keyfile is still alive.