[FIX] http: avoid crash on jsonp POST+GET

- This commit fixes a crash that happens when the server receive a
  JSON-P call done in two requests (a POST followed by a GET).

  The issue is due to the fact that when the first request is done (POST
  one) the member `params` is never initialized.

  This parameter is then used in the module `auth_signup` on an override
  of the method `dispatch` thus crashing the code.

  To avoid the crash, we now initialize `params`.

closes odoo/odoo#27369

odoo/http.py

@@ -578,6 +578,7 @@ class JsonRequest(WebRequest):
         super(JsonRequest, self).__init__(*args)
 
         self.jsonp_handler = None
+        self.params = {}
 
         args = self.httprequest.args
         jsonp = args.get('jsonp')