fix($rootScope.$on) check listener existense while deregistering

Check that listener is still present in $$listeners before decrease
$$listenerCount. It fixes problem with incorrect $$listenerCount after
call deregistering function multiple times.

Closes #9666
Closes #9667

src/ng/rootScope.js

@@ -1103,8 +1103,11 @@ function $RootScopeProvider(){
 
         var self = this;
         return function() {
-          namedListeners[namedListeners.indexOf(listener)] = null;
-          decrementListenerCount(self, 1, name);
+          var indexOfListener = namedListeners.indexOf(listener);
+          if (indexOfListener !== -1) {
+            namedListeners[indexOfListener] = null;
+            decrementListenerCount(self, 1, name);
+          }
         };
       },
 

test/ng/rootScopeSpec.js

@@ -1638,6 +1638,31 @@ describe('Scope', function() {
           expect(child1.$$listenerCount).toEqual({event1: 1});
           expect(child2.$$listenerCount).toEqual({});
         }));
+
+
+        it('should not decrement $$listenerCount when called second time', inject(function($rootScope) {
+          var child = $rootScope.$new(),
+              listener1Spy = jasmine.createSpy(),
+              listener2Spy = jasmine.createSpy();
+
+          child.$on('abc', listener1Spy);
+          expect($rootScope.$$listenerCount).toEqual({abc: 1});
+          expect(child.$$listenerCount).toEqual({abc: 1});
+
+          var deregisterEventListener = child.$on('abc', listener2Spy);
+          expect($rootScope.$$listenerCount).toEqual({abc: 2});
+          expect(child.$$listenerCount).toEqual({abc: 2});
+
+          deregisterEventListener();
+
+          expect($rootScope.$$listenerCount).toEqual({abc: 1});
+          expect(child.$$listenerCount).toEqual({abc: 1});
+
+          deregisterEventListener();
+
+          expect($rootScope.$$listenerCount).toEqual({abc: 1});
+          expect(child.$$listenerCount).toEqual({abc: 1});
+        }));
       });
     });