net: check for canceled context before starting Windows DNS lookup

Fixes #31950

Change-Id: Id9bcd51a8b49523eeecbd0d8d527372a0b8d8760
Reviewed-on: https://go-review.googlesource.com/c/go/+/177038
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/net/lookup_windows.go

@@ -138,11 +138,14 @@ func (r *Resolver) lookupIP(ctx context.Context, network, name string) ([]IPAddr
 		err   error
 	}
 
-	ch := make(chan ret, 1)
-	go func() {
-		addr, err := getaddr()
-		ch <- ret{addrs: addr, err: err}
-	}()
+	var ch chan ret
+	if ctx.Err() == nil {
+		ch = make(chan ret, 1)
+		go func() {
+			addr, err := getaddr()
+			ch <- ret{addrs: addr, err: err}
+		}()
+	}
 
 	select {
 	case r := <-ch: