fix ClassCastException in ZipFile.getInputStream

This was being thrown when the parameter passed was a
JarFile.MyJarEntry, not a ZipFile.MyZipEntry, yet we cast it to the
latter.

classpath/java/util/jar/JarFile.java

@@ -33,7 +33,7 @@ public class JarFile extends ZipFile {
     return (JarEntry) getEntry(JarEntryFactory.Instance, name);
   }
 
-  private static class MyJarEntry extends JarEntry {
+  private static class MyJarEntry extends JarEntry implements MyEntry {
     public final Window window;
     public final int pointer;
 
@@ -65,6 +65,10 @@ public class JarFile extends ZipFile {
         return 0;
       }
     }
+
+    public int pointer() {
+      return pointer;
+    }
   }
 
   private static class JarEntryFactory implements EntryFactory {