[barcode] Bugfix for Barcode_Encode

[Alessandro Rubini]

Hello

> In my first quick little hack, I thought that I could populate the
> Barcode_Item struct myself.  That worked well enough, except that
> when I set BARCODE_NO_CHECKSUM in bc->flags, I got the checksum
> anyway, even though I passed 0 for the flags in Barcode_Encode.
> Here's the snippet of code:

Well found. I'm not sure about your fix, though. With your patch if I
only pass NOHEADERS I loose the encoding from bc->flags and will end
up with no selected encoding. Will see...

/alessandro