sysutils/xfce4-systemload-plugin: Fix crash on properties dialog

Import patch from upstream:

https://git.xfce.org/panel-plugins/xfce4-systemload-plugin/commit/?id=b35286b81e2b6166338008cb9330cf54b0d82250

Approved by: madpilot (xfce)
Differential Revision: https://reviews.freebsd.org/D20672