If a driver has already been quit, do nothing.

We're really only going to make ourselves unhappy if we
try and call `quit` multiple times if the session is
already kaput.

Author: shs96c

java/client/src/org/openqa/selenium/remote/RemoteWebDriver.java

@@ -489,6 +489,11 @@ public void close() {
   }
 
   public void quit() {
+    // no-op if session id is null. We're only going to make ourselves unhappy
+    if (sessionId == null) {
+      return;
+    }
+
     try {
       execute(DriverCommand.QUIT);
     } finally {