Set default value for PhantomJS process reference

In the PhantomJS Service class, the process attribute can be undefined
if something goes wrong.  In that case, the destructor (__del__)
raises an AttributeError.

Signed-off-by: Andreas Tolfsen <ato@mozilla.com>

py/selenium/webdriver/phantomjs/service.py

@@ -48,6 +48,7 @@ def __init__(self, executable_path, port=0, service_args=None, log_path=None):
             self.service_args=service_args[:]
         self.service_args.insert(0, self.path)
         self.service_args.append("--webdriver=%d" % self.port)
+        self.process = None
         if not log_path:
             log_path = "ghostdriver.log"
         self._log = open(log_path, 'w')