Python client: driver.quit() might print <'NoneType' object has no attribute 'path'>

When doing `driver.quit()` the message `'NoneType' object has no attribute 'path'` gets printed.

Testcase:

``` python
from selenium import webdriver
from selenium.webdriver.firefox.firefox_binary import FirefoxBinary

driver = webdriver.Firefox(executable_path="/path/to/geckodriver")

driver.quit()
```

Selenium 3.0.1 for Python 2.
