socks5_connect: Fix compiler warning from GCC9 with -D_FORTIFY_SOURCE=2

    gateway.c: In function 'socks5_connect':
    gateway.c:1249:4: warning: 'strncpy' specified bound 295 equals destination size [-Wstringop-truncation]
     1249 |    strncpy(cbuf + client_pos, target_host, sizeof(cbuf) - client_pos);
          |    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The warning is harmless because 'target_host' is checked to
be at most 255 bytes long.

Reported by Lee.