pohelper: Fallback to a sane default if line break column is too small

There is no point in breaking at a too small column, and the code don't
properly handle length smaller than 2 anyway.  So, use 72 as the
default if the configured line break column has a crazily small value
(less than 8).
pohelper: Explicitly use unsigned length parameter for split_msg()

Use a real unsigned parameter instead of checking the passed-in value
isn't negative.
Merge branch 'pohelper/sane-reflow'