net: fix Windows TestInterfaceHardwareAddrWithGetmac

TestInterfaceHardwareAddrWithGetmac was panicing when getmac returned
multiple network cards.

Change-Id: I6fefa5a4910bce805b4cd9c09f94bd56c9682b9e
Reviewed-on: https://go-review.googlesource.com/47190Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/net_windows_test.go

@@ -592,7 +592,7 @@ func TestInterfaceHardwareAddrWithGetmac(t *testing.T) {
 		addr = strings.Replace(addr, "-", ":", -1)
 		cname := getValue("Connection Name")
 		want[cname] = addr
-		group = nil
+		group = make(map[string]string)
 	}
 	lines := bytes.Split(out, []byte{'\r', '\n'})
 	for _, line := range lines {