[release-branch.go1.10] net/http: lock the read-only mutex in shouldRedirect

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

Fixes #27129

Change-Id: I998448a6e482b5d6a1b24f3354bb824906e23172
Reviewed-on: https://go-review.googlesource.com/95775


Reviewed-by: Andrew Bonventre <andybons@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>