route: use length from message to parse netmask

sizeofSockaddrInet is 16, but first byte of sockaddr specifies the size
of the address. 16 works for most cases, except with Netmasks addresses,
on Darwin where only the significant bits are in the msg.

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 as such.

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

@@ -172,18 +172,31 @@ func (a *Inet6Addr) marshal(b []byte) (int, error) {
 func parseInetAddr(af int, b []byte) (Addr, error) {
 	switch af {
 	case syscall.AF_INET:
-		if len(b) < sizeofSockaddrInet {
+		l := int(b[0])
+		if len(b) < l || l < 4 {
 			return nil, errInvalidAddr
 		}
 		a := &Inet4Addr{}
-		copy(a.IP[:], b[4:8])
+		n := 8
+		if l < 8 {
+			n = l
+		}
+		copy(a.IP[:], b[4:n])
 		return a, nil
 	case syscall.AF_INET6:
-		if len(b) < sizeofSockaddrInet6 {
+		l := int(b[0])
+		if len(b) < l || l < 8 {
 			return nil, errInvalidAddr
 		}
-		a := &Inet6Addr{ZoneID: int(nativeEndian.Uint32(b[24:28]))}
-		copy(a.IP[:], b[8:24])
+		n := 24
+		if l < 24 {
+			n = l
+		}
+		a := &Inet6Addr{}
+		if n >= 28 {
+			a.ZoneID = int(nativeEndian.Uint32(b[24:28]))
+		}
+		copy(a.IP[:], b[8: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