net: fix misrecongnization of IPv6 zone on Windows

Fixes #15463.

Change-Id: Ic85886861c650ffcb71240d847941534152b92bc
Reviewed-on: https://go-review.googlesource.com/22540
Run-TryBot: Mikio Hara <mikioh.mikioh@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/net/interface_windows.go

@@ -158,10 +158,10 @@ func interfaceAddrTable(ifi *Interface) ([]Addr, error) {
 						l = addrPrefixLen(pfx6, IP(sa.Addr[:]))
 					}
 					ifa := &IPNet{IP: make(IP, IPv6len), Mask: CIDRMask(l, 8*IPv6len)}
+					copy(ifa.IP, sa.Addr[:])
 					if ifa.IP.IsLinkLocalUnicast() {
 						ifa.Zone = syscall.UTF16ToString((*(*[10000]uint16)(unsafe.Pointer(aa.FriendlyName)))[:])
 					}
-					copy(ifa.IP, sa.Addr[:])
 					ifat = append(ifat, ifa)
 				}
 			}