[FIX] runbot_merge: typo in freeze wizard

Forgot to deref the id of the staging we're trying to lock, so the
specific case where we start a freeze with a bump PR and an
outstanding staging in master would instantly blow up.

runbot_merge/models/project_freeze/__init__.py

@@ -196,7 +196,7 @@ class FreezeWizard(models.Model):
         if self.bump_pr_ids and master.active_staging_id:
             self.env.cr.execute(
                 'SELECT * FROM runbot_merge_stagings WHERE id = %s FOR UPDATE NOWAIT',
-                [master.active_staging_id]
+                [master.active_staging_id.id]
             )
 
         seq = itertools.count(start=1) # start reseq at 1