...and the column width must not be negative.

The previous column logic ensured that the total width was
larger than the reference by adding a fake buffer
and then subtracted the difference from the final column.
In the case of a zero-width final column, it could become negative.
The current logic ensures that the total width is less than
the reference width, and then adds the difference
(which should be a smaller difference now) to the final column.

Regression potential - early columns that need every single twip
of bonus space might not fit anymore. On the other hand,
ending columns might be fixed...

Change-Id: Ie75d455e8ed62dbec5a1b9c901417df8d842ace8
Reviewed-on: https://gerrit.libreoffice.org/59400
Tested-by: Jenkins
Reviewed-by: Justin Luth <justin_luth@sil.org>