Ensure MemoryClassLoader has a usable constructor

Invoking JNEW-RUNTIME-CLASS before a JavaClassLoader has been
instantiated will fail with a slightly mysterious error message, which
can be fixed by providing a more general constructor.

Fixes <#685>

src/org/armedbear/lisp/MemoryClassLoader.java

@@ -57,6 +57,11 @@ public MemoryClassLoader(JavaClassLoader parent) {
         this.internalNamePrefix = "";
     }
 
+  public MemoryClassLoader(ClassLoader parent) {
+        super(parent);
+        this.internalNamePrefix = "";
+    }
+
     @Override
     protected Class<?> loadClass(String name, boolean resolve)
             throws ClassNotFoundException {