408662 In pax-web servlet services requests even if init() has not finished running

jetty-servlet/src/main/java/org/eclipse/jetty/servlet/ServletHolder.java

@@ -625,6 +625,8 @@ public class ServletHolder extends Holder<Servlet> implements UserIdentity.Scope
         Servlet servlet=_servlet;
         synchronized(this)
         {
+            if (!isStarted())
+                throw new UnavailableException("Servlet not initialized", -1);
             if (_unavailable!=0 || !_initOnStartup)
                 servlet=getServlet();
             if (servlet==null)