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 (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) {