Memory corruption / crash if DPMS extension not available

I have a set that runs with X -dpms to disable power management.

xfce4-power-manager often crashes in memory allocation functions due to corruption. Running under valgrind revealed the culprit being xfpm_dpms_refresh().

What seems to happen is that xfpm_dpms_init() omits the xfconf creation if not DPMS capable. Later on xfpm_dpms_refresh() still gets called, and attempts to use the xfconf. Adding a simple check for dpms_capable in xfpm_dpms_refresh() fixed the issue.

Potentially same issue as #30 (closed)