Since that method uses 'mux.m', we need to lock the mutex to avoid data races.

Change-Id: I998448a6e482b5d6a1b24f3354bb824906e23172
GitHub-Last-Rev: 163a7d4942e793b328e05a7eb91f6d3fdc4ba12b
GitHub-Pull-Request: golang/go#23994
Reviewed-on: https://go-review.googlesource.com/96575Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>