Fix a bug introduced with r249751, in which a small hunk was forgotten in
the performance conversion process.

The effect of this was, when your /etc/defaults/rc.conf file changed
(based on md5(1)) and re-generating the file startup_rcconf_map.cache in
/var/run/bsdconfig/ you would get a screen-dump of its contents before the
menu would appear.
Fix a bug introduced with r251190, in which a small hunk was forgotten in
the performance conversion process.

The effect of this was the following error when selecting the menu
"Startup"->"View/Edit Startup Configuration"->"Add New"->"Add From List":

[: -eq: unexpected operator

By running `bsdconfig -d' as root to enable debugging, this turns into:

DEBUG: f_getvar: var=[text] value=[
Error: Expected a number for token 4 of --menu.
Use --help to list options.] r=0
[: -eq: unexpected operator

Indicating that the fourth token for --menu which should be $height was
instead a string (the first item of $menu_list) because it was using the
old size-calculation method and $size was now null (needed to use instead
the new size variables of $height $width and $rows).