provide default implementation for ClassLoader.reallyFindLoadedClass

This is a non-standard method, so we shouldn't expect subclasses to
implement it.

classpath/java/lang/ClassLoader.java

@@ -54,7 +54,9 @@ public abstract class ClassLoader {
     throw new ClassNotFoundException();
   }
 
-  protected abstract Class reallyFindLoadedClass(String name);
+  protected Class reallyFindLoadedClass(String name) {
+    return null;
+  }
 
   protected final Class findLoadedClass(String name) {
     return reallyFindLoadedClass(name);