fix(uiSelectMultiple): do not add undefined items

Prevent creating and adding undefined items to $select.selected.
Addresses angular-ui#1269.

Author: kavika-1

src/uiSelectMultipleDirective.js

@@ -176,6 +176,9 @@ uis.directive('uiSelectMultiple', ['uiSelectMinErr','$timeout', function(uiSelec
         if($select.selected.length >= $select.limit) {
           return;
         }
+        if (item === undefined) {
+          return;
+        }
         $select.selected.push(item);
         $selectMultiple.updateModel();
       });