views: fix undefined user variable

If negoration needs more than one round-trip, the user variable is
undefined.

src/django_gssapi/views.py

@@ -82,12 +82,13 @@ class LoginView(View):
                 store=self.get_gssapi_store())
         except utils.NegotiateContinue as e:
             token = e.token
+            user = None
 
-        if user is None:
-            response = self.challenge()
-        else:
+        if user is not None:
             logger.debug('GSSAPI found user %s for name %s', user, gss_name)
             response = self.success(user)
+        else:
+            response = self.challenge()
 
         utils.response_add_www_authenticate(response, token)
         return response