jdpack max_bytes is too strict

[pelikhan]

When applying the max_bytes constraint to a string, the jdpack format s[n] complains if the string is shorter than n characters.

    command send_string @ 0x80 {
        message: string { max_bytes=18 }
    }

[mmoskal]

Seems to work for me:

> jacdac.jdpack("s[18]", ["foo"])
[102, 111, 111, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
> jacdac.jdunpack( jacdac.jdpack("s[18]", ["foo"]), "s[18]" )
["foo"]