Do not switch buffer if buffer already switched.

Summary:
Mutt does this. It sets the alternate buffer, it disables it, it
disables it again and enables it back.

Test Plan:
Run mutt, quit mutt, run mutt, quit mutt. You should see the
residual background from mutt.

Reviewers: billiob

Differential Revision: https://phab.enlightenment.org/D371