Avoid dereferencing a null pointer when QMimeData is 0

QClipboard's QPA implementation of clear() calls setMimeData(0, mode),
which means mimeData can be dereferenced when it is null. Prevent this
from happening.

Change-Id: Ib019926e1a7c002d365419db609028c258d364bc
Reviewed-by: Robin Burchell <robin+qt@viroteck.net>