<rdar://problem/4760612>
        TOT REGRESSION: Repro assertion failure when forwarding draft in Range::compareBoundaryPoints
        
        No layout test possible, attributedStringFrom:(DOMNode*)...etc isn't available to DRT.
        
        * dom/Range.cpp:
        (WebCore::Range::boundaryPointsValid): compareBoundaryPoints asserts that the incoming 
        positions are non-null since most of it's callers would be in a bad state if the positions 
        were null.  boundaryPointsValid, which calls compareBoundaryPoints, is an exception.  
        Instead of removing the assert I've added nil checks in boundaryPointsValid, to keep it 
        from entering compareBoundaryPoints and hitting the assert.



git-svn-id: http://svn.webkit.org/repository/webkit/trunk@16740 268f45cc-cd09-0410-ab3c-d52691b4dbfc