QWebPage crashes on drag without QWidget view

QWebPage can be used without a widgets and has a documentation section
on how to do so, but if QWegPage hasn't been given a QWidget in setView
a drag operation will crash. To reproduce select a section of the web page
click and drag from within the selection and it will crash.

Add a check to avoid a NULL dereference, the existing code already has
a check for not having a QWidget set as the view.
DragClientQt::startDrag
m_chromeClient->platformPageClient() is NULL

I have a fix that will be submitted to codereview shortly and I'll add the link in a comment.