base64: fix for unsigned chars (e.g. ARM).

base64: fix for unsigned chars (e.g. ARM).

```
ccan/ccan/base64/base64.c:34:10: error: result of comparison of constant 255 with expression of type 'int8_t' (aka 'signed char') is always false [-Werror,-Wtautological-constant-out-of-range-compare]
        if (ret == (char)0xff) {
            ~~~ ^  ~~~~~~~~~~
ccan/ccan/base64/base64.c:44:57: error: result of comparison of constant 255 with expression of type 'const signed char' is always true [-Werror,-Wtautological-constant-out-of-range-compare]
        return (maps->decode_map[(const unsigned char)b64char] != (char)0xff);
                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ^  ~~~~~~~~~~
```

Reported-by: Christian Decker
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>