gsettings: Fix memory leak on error handling path for g_settings_set()

On the warning/critical error handling paths for g_settings_set(), the
GVariant value was not ref-sunk, and the schema key was leaked. This
won’t affect code in production (unless it’s seriously buggy), but
eliminates some leaks from the error testing paths in the GSettings
tests.

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=779265