Fix the problem of instanceof test always return true.

This instanceof test will always return true because DataType is the superclass of variable nested's class ResourceComparator. The variable nested is not initialized, it would be better to do a null test rather than an instanceof test.
http://findbugs.sourceforge.net/bugDescriptions.html#BC_VACUOUS_INSTANCEOF

src/main/org/apache/tools/ant/types/resources/comparators/Reverse.java

@@ -81,7 +81,7 @@ public class Reverse extends ResourceComparator {
         if (isReference()) {
             super.dieOnCircularReference(stk, p);
         } else {
-            if (nested instanceof DataType) {
+            if (nested != null) {
                 pushAndInvokeCircularReferenceCheck((DataType) nested, stk,
                                                     p);
             }