The function gdk_monitor_get_refresh_rate() can return 0 if there is no
real monitor attached (as with Xephyr or VNC for example).

The compositor use the value as a divisor to compute the frequency of
refresh when zooming, and if the value is 0, xfwm4 will crash with a
floating point exception (division by zero error).

Make sure we return something even if gdk_monitor_get_refresh_rate()
cannot get us a real refresh rate.

Signed-off-by: Olivier Fourdan <fourdan@xfce.org>