net/smtp: ignore HELO error in QUIT

net/smtp: ignore HELO error in QUIT

Fixes #70011

Change-Id: I9d8b3ffbd66561eee0efffd54038960acd5fcf64
Reviewed-on: https://go-review.googlesource.com/c/go/+/622476
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Commit-Queue: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>