data-out: Make each converter responsible for storing null terminator.

data-out: Make each converter responsible for storing null terminator.

Until now, every converter has produced output that is exactly as many
bytes long as the format's width.  In upcoming patches this will change,
because in UTF-8 a character that occupies a single display column can
require multiple bytes.  In preparation, this commit requires the
individual converters to write out their own null byte, giving a bit
more flexibility regarding length.