The current combo completion check only makes sure the last key in the
combo is set. This works when the combo is typed correctly initially, or
when reraising events in a combo of length two. However, it fails for
longer combos since the last event in pressed_keys might be set, but the
first (or subsequent) event in pressed_keys can be NULL thanks to
release_pressed_keys.
Also added a regression test.
4e69a32103