Reviewed by Darin.

    Reviewed by Darin.

	<rdar://problem/3703768> CrashTracer: ...50 crashes at com.apple.WebCore: KHTMLPart::xmlDocImpl const + 0

        * khtml/khtmlview.cpp:
        (KHTMLView::viewportMouseMoveEvent): Add a nil check and an
	assertion for m_part being null. It seems impossible for this to
	happen, so we want to debug it ourselves, but in the meantime,
	let's try to avoid causing crashes for our users.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@7282 268f45cc-cd09-0410-ab3c-d52691b4dbfc