connhelper: fix cmd.Wait() race
connhelper: fix cmd.Wait() race

Fix #1336

Signed-off-by: Akihiro Suda <ea940da7e429af3d0d0c18296cabd0112a36bad5@lab.ntt.co.jp>

cpuguy83/cli,cpuguy83/cli,docker/cli,thaJeztah/cli,docker/cli,thaJeztah/cli