Fix authorized overdraft validation.

accountant/api/models/accounts.py

@@ -111,6 +111,6 @@ class Account(db.Model):
 
     @validates('authorized_overdraft')
     def validate_authorized_overdraft(self, key, authorized_overdraft):
-        assert authorized_overdraft > 0
+        assert authorized_overdraft <= 0
 
         return authorized_overdraft