route: fix address parsing of messages on Darwin

partial fix golang/go#44740

sizeofSockaddrInet is 16, but first byte of sockaddr specifies the size
of the address.

16 works for most cases, except Netmasks addresses, on Darwin where only
the significant bits are in the msg.

Take this route message as an example
```
88 00 05 01 00 00 00 00 41 08 00 00 07 00 00 00 92 7b 00 00 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
DST 10 02 00 00 64 71 00 00 00 00 00 00 00 00 00 00 // 100.113.0.0
GW 14 12 21 00 01 08 00 00 75 74 75 6e 34 33 31 39 00 00 00 00 // !utun4319
MASK 06 02 00 00 ff ff // 255.255.0.0
NULL 00 00
```

i.e. ipv4
```
06 02 00 00 ff ff
```

The above byte sequence is for a sockaddr that is 6 bytes long
representing an ipv4 for address that is 255.255.0.0.

i.e. ipv6 netmask
```
0e 1e 00 00 00 00 00 00 ff ff ff ff ff ff 00 00
```

The above is /48 netmask that should also be parsed using b[0] of the
sockaddr that contains the lenght.

Confirmed by using `route monitor`.

sources:
https://github.com/apple/darwin-xnu/blob/main/bsd/net/route.h
https://github.com/apple/darwin-xnu/blob/main/bsd/sys/socket.h#L603

route/address.go

@@ -170,20 +170,37 @@ func (a *Inet6Addr) marshal(b []byte) (int, error) {
 
 // parseInetAddr parses b as an internet address for IPv4 or IPv6.
 func parseInetAddr(af int, b []byte) (Addr, error) {
+	const (
+		off4 = 4 // offset of in_addr
+		off6 = 8 // offset of in6_addr
+	)
 	switch af {
 	case syscall.AF_INET:
-		if len(b) < sizeofSockaddrInet {
+		if len(b) < (off4+1) || len(b) < int(b[0]) {
 			return nil, errInvalidAddr
 		}
+		sockAddrLen := int(b[0])
 		a := &Inet4Addr{}
-		copy(a.IP[:], b[4:8])
+		n := off4 + 4
+		if sockAddrLen < n {
+			n = sockAddrLen
+		}
+		copy(a.IP[:], b[off4:n])
 		return a, nil
 	case syscall.AF_INET6:
-		if len(b) < sizeofSockaddrInet6 {
+		if len(b) < (off6+1) || len(b) < int(b[0]) {
 			return nil, errInvalidAddr
 		}
-		a := &Inet6Addr{ZoneID: int(nativeEndian.Uint32(b[24:28]))}
-		copy(a.IP[:], b[8:24])
+		sockAddrLen := int(b[0])
+		n := off6 + 16
+		if sockAddrLen < n {
+			n = sockAddrLen
+		}
+		a := &Inet6Addr{}
+		if sockAddrLen == sizeofSockaddrInet6 {
+			a.ZoneID = int(nativeEndian.Uint32(b[24:28]))
+		}
+		copy(a.IP[:], b[off6:n])
 		if a.IP[0] == 0xfe && a.IP[1]&0xc0 == 0x80 || a.IP[0] == 0xff && (a.IP[1]&0x0f == 0x01 || a.IP[1]&0x0f == 0x02) {
 			// KAME based IPv6 protocol stack usually
 			// embeds the interface index in the