Fix appending to a non-grouped multi

When the multi is not grouped, just set the value and do not try to
update len of inexistant slaves.

The code path try to update the len of all other variables under the
OptionDescription of the multi.

* tiramisu/option.py (Multi.append): return after setting the value.

Fixes: #4811 @10m

tiramisu/option.py

@@ -79,7 +79,10 @@ class Multi(list):
                 raise IndexError("in a group with a master, you mustn't add "
                         "a value in a slave's Multi value")
         except TypeError:
+            # Not a master/slaves
             self._setvalue(value, who=settings.get_owner())
+            return
+
         multis = []
         for opt in self.config._cfgimpl_descr._children:
             if isinstance(opt, OptionDescription):