gdatetime: Fix overflow checks when constructing from timestamps

GDateTime does overflow checks to see if the timestamp being passed in
is too big to be represented. However, it only does those after
converting from a timestamp to an interval, which involves some
multiplications and additions — and hence can overflow, and cause the
later bounds check to erroneously succeed. This results in a non-NULL
GDateTime being returned which represents completely the wrong date.

Fix the overflow checks (do them earlier) and add some unit tests.

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

https://bugzilla.gnome.org/show_bug.cgi?id=782089