Unix socket: Path length check directly before copying the path.

This is not needed as the string is always short enough, anyway
it may be needed in future and one strlen during BIRD start is
cheap enough.

sysdep/unix/io.c

@@ -1495,7 +1495,9 @@ sk_open_unix(sock *s, char *name)
   if (fcntl(fd, F_SETFL, O_NONBLOCK) < 0)
     return -1;
 
-  /* Path length checked in test_old_bird() */
+  /* Path length checked in test_old_bird() but we may need unix sockets for other reasons in future */
+  ASSERT_DIE(strlen(name) < sizeof(sa.sun_path));
+
   sa.sun_family = AF_UNIX;
   strcpy(sa.sun_path, name);