internal/socks: fix socket descriptor leakage in Dialer.Dial
internal/socks: fix socket descriptor leakage in Dialer.Dial

Fixes golang/go#27630.

Change-Id: Id44ade435733ffcd2fc8159ea9ea3287baadf97a
Reviewed-on: https://go-review.googlesource.com/134755
Run-TryBot: Mikio Hara <e2f4d2eec1d9a1f781345cdd7260d7d52e6468ae@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@golang.org>

golang/net