Do not try to load from jars which don't exist

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@267907 13f79535-47bb-0310-9956-ffa450edef68

src/main/org/apache/tools/ant/AntClassLoader.java

@@ -178,6 +178,10 @@ public class AntClassLoader  extends ClassLoader {
      */
     private InputStream getResourceStream(File file, String resourceName) {
         try {
+            if (!file.exists()) {
+                return null;
+            }
+            
             if (file.isDirectory()) {
                 File resource = new File(file, resourceName); 
                 if (resource.exists()) {