Allow stop to be safely called multiple times.

If stop is called on an already-stopped service (e.g. by __del__) it must not signal the PID of the now-dead phantomjs process.

Signed-off-by: AutomatedTester <dburns@mozilla.com>

py/selenium/webdriver/phantomjs/service.py

@@ -106,6 +106,7 @@ def stop(self):
                 self.process.stdin.close()
                 self.process.kill()
                 self.process.wait()
+                self.process = None
         except OSError:
             # kill may not be available under windows environment
             pass