effects/private: Add another guard for multi-screen Drag&Drop

Unlike D&D actions, this method does not return any feedback as to
whether an item drop was accepted by any other screen, and so whether it
is going through destruction process already, so the best we can do is
add another typeof/undefined check after calling that method.

Fixes the following error:

    file:///usr/lib/qml/org/kde/kwin/private/effects/WindowHeapDelegate.qml:415:
    TypeError: Cannot read property 'deleteDND' of undefined