gdatetime: Fix a leak in g_date_time_new_week()

This was a small leak of a GDateTime instance from an internal helper
function, which was using it to calculate week numbers, and then forgot
to free it.

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

https://bugzilla.gnome.org/show_bug.cgi?id=795165