Summary:
Libinput returns -1 when calling libinput_event_touch_get_slot on an event
from a single-touch touchscreen. The returned value is used in
DecorationEventFilter to determine the touch which is acting on it. The value -1
is used to signify that the decoration is not being acted on. Thus when
releasing the touch, it checks whether it was being dragged, and as it thinks
it isn't, it doesn't handle it and the decoration is still being dragged.
Clicking on the decoration then crashes kwin.

Test Plan: Move a window by dragging the decoration with a single-touch touchscreen.

Reviewers: #kwin, graesslin

Reviewed By: #kwin, graesslin

Subscribers: graesslin, kwin, #kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D7854