Attempt fix #2 for cellsAroundViewport reaching out of bounds

Summary: VirtualizedList would more gracefully handle out of range cells than VirtualizedList_EXPERIMENTAL, which treats it as an invariant violation. D39244112 (facebook@7aa203b) attempted to fix an issue where recalculation of cells around viewport can include out of range cells, but it is still showing up later. This change adds a bounds check to the remaining branch we control, and an assertion that `computeWindowedRenderLimits` is not returing something out of range to discover if that is the cause.

Reviewed By: yungsters

Differential Revision: D39267445

fbshipit-source-id: 64c99da28b5b01ef61784079b586e355f73764a1

Libraries/Lists/VirtualizedList_EXPERIMENTAL.js

@@ -608,7 +608,9 @@ class VirtualizedList extends StateSafePureComponent<Props, State> {
     // Wait until the scroll view metrics have been set up. And until then,
     // we will trust the initialNumToRender suggestion
     if (visibleLength <= 0 || contentLength <= 0) {
-      return cellsAroundViewport;
+      return cellsAroundViewport.last >= getItemCount(data)
+        ? VirtualizedList._constrainToItemCount(cellsAroundViewport, props)
+        : cellsAroundViewport;
     }
 
     let newCellsAroundViewport: {first: number, last: number};
@@ -654,6 +656,10 @@ class VirtualizedList extends StateSafePureComponent<Props, State> {
         this.__getFrameMetricsApprox,
         this._scrollMetrics,
       );
+      invariant(
+        newCellsAroundViewport.last < getItemCount(data),
+        'computeWindowedRenderLimits() should return range in-bounds',
+      );
     }
 
     if (this._nestedChildLists.size > 0) {