route: fix address parsing of messages on Darwin

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

Although, 16 works for most cases, it fails for Netmasks addresses. On
Darwin only the significant bits of the netmask are in the msg.

Take this route message as an example

```
// rt_msg_hdr
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

// metrics
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

// SOCKADDRS - DST  (100.113.0.0)
10 02 00 00 64 71 00 00
00 00 00 00 00 00 00 00

// GW  utun4319
14 12 21 00 01 08 00 00
75 74 75 6e 34 33 31 39
00 00 00 00

// NETMASK 255.255.0.0
06 02 00 00 ff ff

// 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 length.

Confirmed by using `route monitor`.

Fixes golang/go#44740

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

Author: hurricanehrndz

Diff for: 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