[FIX] gamification: initialize goals at 0

If a manual goal has a target of 10, it should not start at 9 but 0
If it has a target of 0, it should start at -1

Task id: 2057898

addons/gamification/models/challenge.py

@@ -383,9 +383,9 @@ class Challenge(models.Model):
 
                     # the goal is initialised over the limit to make sure we will compute it at least once
                     if line.condition == 'higher':
-                        values['current'] = line.target_goal - 1
+                        values['current'] = min(line.target_goal - 1, 0)
                     else:
-                        values['current'] = line.target_goal + 1
+                        values['current'] = max(line.target_goal + 1, 0)
 
                 if challenge.remind_update_delay:
                     values['remind_update_delay'] = challenge.remind_update_delay