Avoid getting an NPE by making sure the property attr is set.

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@272810 13f79535-47bb-0310-9956-ffa450edef68

src/main/org/apache/tools/ant/taskdefs/UpToDate.java

@@ -223,6 +223,10 @@ public class UpToDate extends Task implements Condition {
      * than (each of) the corresponding source file(s).
      */
     public void execute() throws BuildException {
+        if (_property == null) {
+            throw new BuildException("property attribute is required.", 
+                                     location);
+        }
         boolean upToDate = eval();
         if (upToDate) {
             this.project.setNewProperty(_property, getValue());