NPE in cleanup on Windows (all non-Unix?).

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@276626 13f79535-47bb-0310-9956-ffa450edef68

src/testcases/org/apache/tools/ant/taskdefs/optional/unix/SymlinkTest.java

@@ -182,7 +182,9 @@ public class SymlinkTest extends BuildFileTest {
     }
 
     public void tearDown() {
-        executeTarget("teardown");
+        if (supportsSymlinks) {
+            executeTarget("teardown");
+        }
     }
 
 }