Guard data source being deleted before processing dataoffer receive event

Summary:
A data source can disappear at any moment, it's plausible a client could
have requested data whilst the client was alive and we are processing it
afterwards.

The accept and finish callback guard against the source being deleted,
but recieve did not.

BUG: 400311

Test Plan: Could still copy/paste as before

Reviewers: KWin, romangg

Reviewed By: KWin, romangg

Subscribers: kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D16431