guard against no comparators on circular ref. check

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@278501 13f79535-47bb-0310-9956-ffa450edef68

src/main/org/apache/tools/ant/types/resources/Sort.java

@@ -163,11 +163,13 @@ public class Sort extends BaseResourceCollectionWrapper {
         if (isReference()) {
             super.dieOnCircularReference(stk, p);
         } else {
-            for (Iterator i = comp.v.iterator(); i.hasNext();) {
-                Object o = i.next();
-                if (o instanceof DataType) {
-                    stk.push(o);
-                    invokeCircularReferenceCheck((DataType) o, stk, p);
+            if (comp.v != null && comp.v.size() > 0) {
+                for (Iterator i = comp.v.iterator(); i.hasNext();) {
+                    Object o = i.next();
+                    if (o instanceof DataType) {
+                        stk.push(o);
+                        invokeCircularReferenceCheck((DataType) o, stk, p);
+                    }
                 }
             }
             setChecked(true);