Ignore AttributeError when shutting down server

It's possible for the _server attribute to not exist if make_server
raises an error in the constructor.

Author: tjni

pytest_localserver/http.py

@@ -31,7 +31,10 @@ def __del__(self):
         self.stop()
 
     def stop(self):
-        self._server.shutdown()
+        try:
+            self._server.shutdown()
+        except AttributeError:
+            pass
 
     @property
     def url(self):