Fix memory leak when QQuickPixmapReply::Event is delete before being used

Fix memory leak when QQuickPixmapReply::Event is delete before being used

It can happen that QQuickPixmapReply::postReply is called, and before
QQuickPixmapReply::event is called the object gets deleted. That means that
the texture factory will never be deleted.

To fix that we delete it in the destructor of QQuickPixmapReply::Event and set
it to 0 in QQuickPixmapReply::event after assigning to the next data structure
that will take care of it

Change-Id: Ibea62f5a10a53cca586de7c5f03f00aabfb88b2e
Reviewed-by: Robin Burchell <robin.burchell@viroteck.net>