Max IPv6 address length is 45 characters, not 16

The idea was to reserve 16 bytes for an IPv6 address. But given that a human representation is used rather than a
128 bit integer, we need 45 characters at most.

gomilter.go

@@ -212,12 +212,12 @@ func Go_xxfi_connect(ctx *C.SMFICTX, hostname *C.char, hostaddr *C._SOCK_ADDR) C
 
 	} else if hostaddr.sa_family == C.AF_INET6 {
 		sa_in := (*C.struct_sockaddr_in6)(unsafe.Pointer(hostaddr))
-		var dst = make([]byte, 16)
+		var dst = make([]byte, 45)
 		C.inet_ntop(
 			C.int(hostaddr.sa_family),
 			unsafe.Pointer(&sa_in.sin6_addr),
 			(*C.char)(unsafe.Pointer(&dst)),
-			16)
+			45)
 
 		ip = net.ParseIP(C.GoString((*C.char)(unsafe.Pointer(&dst))))
 	} else {