[Navigator: Prevent user from over-popping the routes.

Summary:
If user taps the back button quickly, the app crashes becuase "pop"
internally only checks `this.state.presentedIndex` which does not
always update when transtion happens.

This diff addresses this issue.

Libraries/CustomComponents/Navigator/Navigator.js

@@ -922,7 +922,19 @@ var Navigator = React.createClass({
   },
 
   pop: function() {
+    if (this.state.transitionQueue.length) {
+      // This is the workaround to prevent user from firing multiple `pop()`
+      // calls that may pop the routes beyond the limit.
+      // Because `this.state.presentedIndex` does not update until the
+      // transition starts, we can't reliably use `this.state.presentedIndex`
+      // to know whether we can safely keep popping the routes or not at this
+      //  moment.
+      return;
+    }
+
+    if (this.state.presentedIndex > 0) {
       this._popN(1);
+    }
   },
 
   /**