Issue #5365 ISE not NPE if fail to create session.

If SessionHandler.newHttpSession(Request) fails to create a session
it returns null. Request.getSession(true) cannot throw a checked
exception, nor can it return null, so we should throw ISE.

Signed-off-by: Jan Bartel <janb@webtide.com>

jetty-server/src/main/java/org/eclipse/jetty/server/Request.java

@@ -1623,6 +1623,9 @@ public HttpSession getSession(boolean create)
             throw new IllegalStateException("No SessionManager");
 
         _session = _sessionHandler.newHttpSession(this);
+        if (_session == null)
+            throw new IllegalStateException("Create session failed");
+
         HttpCookie cookie = _sessionHandler.getSessionCookie(_session, getContextPath(), isSecure());
         if (cookie != null)
             _channel.getResponse().replaceCookie(cookie);