control/controlclient: early return and outdent some code

I found this too hard to read before.

This is pulled out of #12033 as it's unrelated cleanup in retrospect.

Updates #12028

Change-Id: I727c47e573217e3d1973c5b66a76748139cf79ee
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f