There might be some inconsistent event for which we don't have a known
source device.
In the current state we don't handle them and we could crash when getting
the current device tool.
So, add an utility function that retrieves the source device for an event
that warns if no device is found, and use this for Motion, Key and Button
events.
In case we don't have a valid source in such case, just return early instead
of trying to generate invalid clutter events.
https://gitlab.gnome.org/GNOME/mutter/merge_requests/823
427670cc63