Fix Js error in Navigator

Summary: public

In Navigator, there are several places that mutate `this.state.presentedIndex`
with the express `this.state.presentedIndex = destIndex` instead of calling
`this.setState`, which creates the problem that not all internal states are
updated within the same React update cycle.

One of the symptoms is that over-swiping within the Navigator may throw JS error
due to `this.state.sceneConfigStack` and `this.state.presentedIndex` are not both
updated.

The workaround is to bypass the over-swiping gesture to avoid JS error.

Reviewed By: ericvicenti

Differential Revision: D2557140

fb-gh-sync-id: 1e5c9047ed17c04a63e2a568118848b00723fb1d

Libraries/CustomComponents/Navigator/Navigator.js

@@ -591,6 +591,9 @@ var Navigator = React.createClass({
 
   _handleMoveShouldSetPanResponder: function(e, gestureState) {
     var sceneConfig = this.state.sceneConfigStack[this.state.presentedIndex];
+    if (!sceneConfig) {
+      return false;
+    }
     this._expectingGestureGrant = this._matchGestureAction(this._eligibleGestures, sceneConfig.gestures, gestureState);
     return !!this._expectingGestureGrant;
   },