control/controlclient: fix log print with always-empty key

In debugging #4541, I noticed this log print was always empty.
The value printed was always zero at this point.

Updates #4541

Change-Id: I0eef60c32717c293c1c853879446be65d9b2cef6
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f