Guard FlatList getItemCount() against incorrect data type

Summary:
If FlatList is passed non-array data it will return `undefined` as the result of `getItemCount()` to VirtualizedList. This change makes it return `0` instead, to signify there are no valid items to attempt to accces.

There are a set of invariants on properties passed to FlatList, to curb incorrect types at runtime, but there is existing code which runs into the condition.

Changelog:
[Internal][Fixed] - Guard FlatList getItemCount() against non-array data

Reviewed By: javache

Differential Revision: D38198351

fbshipit-source-id: 9efd0df7eeeba17078e2c838d470c4b0d621b9a0

Libraries/Lists/FlatList.js

@@ -511,7 +511,7 @@ class FlatList<ItemT> extends React.PureComponent<Props<ItemT>, void> {
   };
 
   _getItemCount = (data: ?Array<ItemT>): number => {
-    if (data) {
+    if (Array.isArray(data)) {
       const numColumns = numColumnsOrDefault(this.props.numColumns);
       return numColumns > 1 ? Math.ceil(data.length / numColumns) : data.length;
     } else {