Avoid null pointer by checking to make sure extension is non-null before adding it to manifest

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@272252 13f79535-47bb-0310-9956-ffa450edef68

proposal/myrmidon/src/java/org/apache/antlib/extensions/JarLibManifestTask.java

@@ -184,7 +184,10 @@ public final class JarLibManifestTask
 
         appendExtraAttributes( attributes );
 
-        Extension.addExtension( m_extension, attributes );
+        if( null != m_extension )
+        {
+            Extension.addExtension( m_extension, attributes );
+        }
 
         //Add all the dependency data to manifest for dependencies
         final ArrayList depends = toExtensions( m_dependencies );