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.

pkg/arvo/lib/hood/drum.hoon

@@ -564,7 +564,12 @@
 ::
 ++  se-klin                                           ::  disconnect app
   |=  gyl=gill:gall
-  +>(eel (~(del in eel) gyl))
+  =/  gil=(unit gill:gall)  se-agon
+  =.  eel  (~(del in eel) gyl)
+  ?~  gil  +>.$
+  ?:  =(gyl u.gil)
+    +>.$(inx 0)
+  (se-alas u.gil)
 ::
 ++  se-link                                           ::  connect to app
   |=  gyl=gill:gall