g_settings_schema_list: some fixes

Prevent a crash in the case that gvdb_table_list() returns NULL (ie:
because a schema has no keys).

Stop a memory leak caused by pointlessly stealing keys from a hashtable
(after we quarked them already).

Stop allocating an extra entry at the end of an array for a terminator
(that we never wrote anyway) when all functions using this API refer to
the out-parameter length array.

https://bugzilla.gnome.org/show_bug.cgi?id=711016