crypto/tls: stop a timeout timer

I noticed this leak while writing CL 214977.

Change-Id: I7566952b8e4bc58939d23435aea86576fc58ddca
Reviewed-on: https://go-review.googlesource.com/c/go/+/214978
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/crypto/tls/tls.go

@@ -116,9 +116,10 @@ func DialWithDialer(dialer *net.Dialer, network, addr string, config *Config) (*
 
 	if timeout != 0 {
 		errChannel = make(chan error, 2)
-		time.AfterFunc(timeout, func() {
+		timer := time.AfterFunc(timeout, func() {
 			errChannel <- timeoutError{}
 		})
+		defer timer.Stop()
 	}
 
 	rawConn, err := dialer.Dial(network, addr)