drum: update app index early when unlinking
drum: update app index early when unlinking

The added logic also happens in +se-drop, which will get called by
+se-abet if we did unlink an application. But +se-agon depends on the
index being sane, and may be called between +se-klin and +se-abet.

ngzax/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,urbit/urbit,urbit/urbit,ngzax/urbit,ngzax/urbit,ngzax/urbit,urbit/urbit,urbit/urbit,ngzax/urbit,urbit/urbit,urbit/urbit