[New Multicolumn] Guard against zero or negative space shortage.

We need positive values in order to get anywhere when stretching columns
in order to balance them, and we may get called with zero or negative
values when there's zero-height content at column boundaries, or when we
set an early break in order to honor widows in the next column.

BUG=369985

Review URL: https://codereview.chromium.org/263203003

git-svn-id: svn://svn.chromium.org/blink/trunk@174088 bbb929c8-8fbe-4397-9dbb-9b2b20218538