[FIX] survey: avoid navigation crash

If a survey has "allow users to go back" option enabled, users can go back to previous pages and change their answers before submitting the whole survey.

This commit fixes the very special case, when the user reaches the last page of the survey, then click on "Previous" button, then reopen the survey from the invitation URL (/survey/fill/<survey_id>/<token> without the /prev flag in the URL). This won't crash anymore.

This commit fixes odoo#2658 and odoo#2680.

addons/survey/controllers/main.py

@@ -166,6 +166,11 @@ def fill_survey(self, survey, token, prev=None, **post):
         elif user_input.state == 'skip':
             flag = (True if prev and prev == 'prev' else False)
             page, page_nr, last = survey_obj.next_page(cr, uid, user_input, user_input.last_displayed_page_id.id, go_back=flag, context=context)
+
+            #special case if you click "previous" from the last page, then leave the survey, then reopen it from the URL, avoid crash
+            if not page:
+                page, page_nr, last = survey_obj.next_page(cr, uid, user_input, user_input.last_displayed_page_id.id, go_back=True, context=context)
+
             data = {'survey': survey, 'page': page, 'page_nr': page_nr, 'token': user_input.token}
             if last:
                 data.update({'last': True})