Fix enable static NAT dialog

Require VM selection, or pop-up error message -- prevents server call
from being made without a required VM.

ui/scripts/ui-custom/enableStaticNAT.js

@@ -56,6 +56,14 @@
             text: 'Apply',
             'class': 'ok',
             click: function() {
+              if (!$dataList.find(
+                'input[type=radio]:checked, input[type=checkbox]:checked'
+              ).size()) {
+                cloudStack.dialog.notice({ message: 'Please select an instance '});
+                
+                return false;
+              }
+
               var complete = args.complete;
               
               $dataList.fadeOut(function() {