sanity check filter tokens.

PR: 4634


git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@269908 13f79535-47bb-0310-9956-ffa450edef68

src/main/org/apache/tools/ant/types/FilterSet.java

@@ -245,6 +245,9 @@ public class FilterSet extends DataType {
         if (isReference()) {
             throw tooManyAttributes();
         }
+        if (startOfToken == null || "".equals(startOfToken)) {
+            throw new BuildException("beginToken must not be empty");
+        }
         this.startOfToken = startOfToken;
     }
 
@@ -265,6 +268,9 @@ public class FilterSet extends DataType {
         if (isReference()) {
             throw tooManyAttributes();
         }
+        if (endOfToken == null || "".equals(endOfToken)) {
+            throw new BuildException("endToken must not be empty");
+        }
         this.endOfToken = endOfToken;
     }