Fix parameter checking when setting bucket-pointing policy

Fix parameter checking when setting bucket-pointing policy

Field result_extra in cynara_admin_policy structure should contain
another bucket's name in case of insertion or update of a policy
pointing to another bucket.

There was no check for that. In case a nullptr was passed, it was
converted to an empty string "".

Now cynara_admin_set_policies
returns CYNARA_ADMIN_API_INVALID_PARAM when
result is set to CYNARA_ADMIN_BUCKET and
result_extra is nullptr.

Change-Id: I24ff5ab662e88b7bc538368385e13d78f48f9e9a