Potential fix for #44740

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.
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.

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

@@ -172,7 +172,8 @@ 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 {
 			return nil, errInvalidAddr
 		}
 		a := &Inet4Addr{}