Crash at WebCore::SharedBuffer::hasPlatformData during paste.
https://bugs.webkit.org/show_bug.cgi?id=111207.
<rdar://problem/13024528>

Reviewed by Ryosuke Niwa.

This is a speculative fix since we don't have a solid repro case.
Adding null check every time we get a SharedBuffer from the pasteboard.

* platform/mac/PasteboardMac.mm:
(WebCore::Pasteboard::plainText):
(WebCore::documentFragmentWithRTF):



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@144479 268f45cc-cd09-0410-ab3c-d52691b4dbfc