Fix a bug introduced with r251190, in which a small hunk was forgotten in

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).