[server] Do not return early on fail in touchDown fall back code

Summary:
Returning early would mean that SeatInterface::Private::touchInterface.ids
does not contain the id. Then the assert would fail in touchUp at some
point later when called by the compositor.

Reviewers: KWin, Frameworks, davidedmundson

Reviewed By: KWin, davidedmundson

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D15468