If a testing.TB is no longer on the stack, t.Log would panic because
its outputWriter is nil. Check for nil and drop the write, which
is the previous behavior.
Change-Id: Ifde97997a3aa26ae604ac9c218588c1980110cbf
Reviewed-on: https://go-review.googlesource.com/c/go/+/673215
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com> Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Jonathan Amsterdam <jba@google.com>