MIN Perform cooperative yield before checking flag

Previously, it was possible that we would perform an extra (unnecessary)
loop.

gui/tasks.py

@@ -188,8 +188,8 @@ def _perform(self):
             messages = [Message.load_by_mid(m, create_session=(lambda:session)) for m in ms]
             rbglobals.index.add(messages)
             session.expunge_all()
+            sleep()
             if self.dead:
                 break
-            sleep()
         self.status.emit('Message reindexing complete')