Fix NPE in UsrGrpAdminServlet

In commit 11d268f the
UsrGrpAdminServlet's constructor was modified to get the CMSEngine
object from the servletContext before the field was initialized
which triggered a NullPointerException. To fix the problem the code
in the constructor has been moved into init() so that it will run
after the servletContext initialization.

https://issues.redhat.com/browse/RHEL-626

base/server/src/main/java/com/netscape/cms/servlet/admin/UsrGrpAdminServlet.java

@@ -87,22 +87,14 @@ public class UsrGrpAdminServlet extends AdminServlet {
     private static final String MULTI_ROLE_ENABLE = "multiroles.enable";
     private static final String MULTI_ROLE_ENFORCE_GROUP_LIST = "multiroles.false.groupEnforceList";
 
-    /**
-     * Constructs User/Group manager servlet.
-     */
-    public UsrGrpAdminServlet() {
-        super();
-        CMSEngine engine = getCMSEngine();
-        mAuthz = engine.getAuthzSubsystem();
-    }
-
     /**
      * Initializes this servlet.
      */
     @Override
     public void init(ServletConfig config) throws ServletException {
         super.init(config);
         CMSEngine engine = getCMSEngine();
+        mAuthz = engine.getAuthzSubsystem();
         mMgr = engine.getUGSubsystem();
     }