This could produce a confusing panic message if the second len(g.sem)
call returns 0. Avoid that by reading it once.

Change-Id: Ibdb4963f90921bc20427b3f1e2de410638f6cb6b
Reviewed-on: https://go-review.googlesource.com/c/sync/+/726280
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
Reviewed-by: Sean Liao <sean@liao.dev>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>