[Qt] editing/pasteboard/can-read-in-dragstart-event.html and /can-read-in-copy-and-cut-events.html are crashing
https://bugs.webkit.org/show_bug.cgi?id=113126

Reviewed by Ryosuke Niwa.

Source/WebCore:

The ClipboardQt implementation only allows reading or writing, not both. Attempting to read
when the clipboard is only writable will lead to a crash since the corresponding member will
be null. To prevent crashes, change the asserts to early returns. In the long term, the
correct fix is to unify the m_readableData and m_writableData members.

No new tests since no functionality in Qt port should change.

* platform/qt/ClipboardQt.cpp:
(WebCore::ClipboardQt::getData):
(WebCore::ClipboardQt::types):
(WebCore::ClipboardQt::files):

LayoutTests:

* platform/qt/TestExpectations:

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@146723 268f45cc-cd09-0410-ab3c-d52691b4dbfc