This fixes a race which made it possible to cancel a connection after
returning from net.Dial.

Fixes #15035
Fixes #15078

Change-Id: Iec6215009538362f7ad9f408a33549f3e94d1606
Reviewed-on: https://go-review.googlesource.com/21497Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>