Update EventEmitter.js (#23047)

Summary:
added protection for emit() because subscription.listener may have been removed during event loop

Changelog:
----------

[GENERAL][FIX] Avoid crash when subscription listener may have been removed during event loop
Pull Request resolved: https://github.com/facebook/react-native/pull/23047

Differential Revision: D13761725

Pulled By: cpojer

fbshipit-source-id: 0753b780067bd712cd43caf63020c3f0d6ea9f52

Libraries/vendor/emitter/EventEmitter.js

@@ -176,7 +176,7 @@ class EventEmitter {
         const subscription = subscriptions[i];
 
         // The subscription may have been removed during this event loop.
-        if (subscription) {
+        if (subscription && subscription.listener) {
           this._currentSubscription = subscription;
           subscription.listener.apply(
             subscription.context,