fix(carousel): ensure there are slides present

- Ensure there are slides present before calling `$scope.next` in timer

Fixes #3755

Author: wesleycho

src/carousel/carousel.js

@@ -113,7 +113,7 @@ angular.module('ui.bootstrap.carousel', [])
 
   function timerFn() {
     var interval = +$scope.interval;
-    if (isPlaying && !isNaN(interval) && interval > 0) {
+    if (isPlaying && !isNaN(interval) && interval > 0 && slides.length) {
       $scope.next();
     } else {
       $scope.pause();

src/carousel/test/carousel.spec.js

@@ -146,7 +146,7 @@ describe('carousel', function() {
       testSlideActive(1);
     });
 
-    it('shouldnt go forward if interval is NaN or negative', function() {
+    it('shouldnt go forward if interval is NaN or negative or has no slides', function() {
       testSlideActive(0);
       var previousInterval = scope.interval;
       scope.$apply('interval = -1');
@@ -161,6 +161,9 @@ describe('carousel', function() {
       scope.$apply('interval = 1000');
       $interval.flush(1000);
       testSlideActive(2);
+      scope.$apply('slides = []');
+      $interval.flush(1000);
+      testSlideActive(2);
     });
 
     it('should bind the content to slides', function() {