fix(input): check that containerCtrl.label is defined before accessing it

src/components/input/input.js

@@ -474,9 +474,11 @@ function inputTextareaDirective($mdUtil, $window, $mdAria, $timeout, $mdGesture)
     function setupAttributeWatchers() {
       if (containerCtrl.label) {
         attr.$observe('required', function (value) {
-          // We don't need to parse the required value, it's always a boolean because of angular's
+          // We don't need to parse the required value, it's always a boolean because of AngularJS'
           // required directive.
-          containerCtrl.label.toggleClass('md-required', value && !mdNoAsterisk);
+          if (containerCtrl.label) {
+            containerCtrl.label.toggleClass('md-required', value && !mdNoAsterisk);
+          }
         });
       }
     }