Cherry-pick 0c592c9. rdar://105887621

    Crash under ScrollingStateFixedNode::reconcileLayerPositionForViewportRect
    https://bugs.webkit.org/show_bug.cgi?id=270361
    rdar://105887621

    Reviewed by Simon Fraser.

    We need a similar fix to #17453 as there is
    the same crash signature under ScrollingStateFixedNode::reconcileLayerPositionForViewportRect
    this time.

    * Source/WebCore/page/scrolling/ScrollingStateFixedNode.cpp:
    (WebCore::ScrollingStateFixedNode::reconcileLayerPositionForViewportRect):

    Canonical link: https://commits.webkit.org/275574@main

Identifier: 272448.679@safari-7618-branch

Source/WebCore/page/scrolling/ScrollingStateFixedNode.cpp

@@ -84,6 +84,10 @@ void ScrollingStateFixedNode::reconcileLayerPositionForViewportRect(const Layout
     FloatPoint position = m_constraints.layerPositionForViewportRect(viewportRect);
     if (layer().representsGraphicsLayer()) {
         auto* graphicsLayer = static_cast<GraphicsLayer*>(layer());
+        ASSERT(graphicsLayer);
+        // Crash data suggest that graphicsLayer can be null: rdar://105887621.
+        if (!graphicsLayer)
+            return;
 
         LOG_WITH_STREAM(Scrolling, stream << "ScrollingStateFixedNode " << scrollingNodeID() <<" reconcileLayerPositionForViewportRect " << action << " position of layer " << graphicsLayer->primaryLayerID() << " to " << position);