net: don't allocate dialer closure if we won't use it

When we use dialMulti we also allocate dialSingle closure for no reason.

Change-Id: I074282a9d6e2c2a1063ab311a1b95e10fe65219f
Reviewed-on: https://go-review.googlesource.com/4119Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/dial.go

@@ -159,13 +159,15 @@ func (d *Dialer) Dial(network, address string) (Conn, error) {
 	if err != nil {
 		return nil, &OpError{Op: "dial", Net: network, Addr: nil, Err: err}
 	}
-	dialer := func(deadline time.Time) (Conn, error) {
-		return dialSingle(network, address, d.LocalAddr, ra.toAddr(), deadline)
-	}
+	var dialer func(deadline time.Time) (Conn, error)
 	if ras, ok := ra.(addrList); ok && d.DualStack && network == "tcp" {
 		dialer = func(deadline time.Time) (Conn, error) {
 			return dialMulti(network, address, d.LocalAddr, ras, deadline)
 		}
+	} else {
+		dialer = func(deadline time.Time) (Conn, error) {
+			return dialSingle(network, address, d.LocalAddr, ra.toAddr(), deadline)
+		}
 	}
 	c, err := dial(network, ra.toAddr(), dialer, d.deadline())
 	if d.KeepAlive > 0 && err == nil {