[cv] fix panic in servicecfg.
[cv] fix panic in servicecfg.

You can guess how it was discovered :)

R=yiwzhang

Change-Id: I3c1c5e913682cb9ca054d94222c7f7407e95734a
Reviewed-on: https://chromium-review.googlesource.com/c/infra/luci/luci-go/+/2803253
Reviewed-by: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>
Commit-Queue: Yiwei Zhang <50b2a565e2e78c292794832469a30ce5abc9959c@google.com>
Auto-Submit: Andrii Shyshkalov <a30c74fa30536fe7ea81ed6dec202e35e149e1fd@google.com>

luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go,luci/luci-go