dialog-1.3-20180621 displays ugly black bars on FreeBSD

[Alan Somers]

Prior to 1.3-20180621 dialog would fill unused space at the end of the
line with the background color.  However, starting with 1.3-20180621
it doesn't.  Instead, it seems to fill with whatever the terminal's
background color was prior to starting dialog.

Steps to reproduce on FreeBSD current:
cat /etc/rc.conf | dialog --programbox 23 76

With older versions of dialog the box's background is a consistent
color.  With 1.3-20180621 and newer, the trailing unused part of each
line is filled with (on my terminal at least) black.

The problem is evident in FreeBSD's bundled version of dialog, in the
ports tree's version, and in the upstream sources.

-Alan