xfce4-panel: binary clock leaks memory

When adding a binary clock in non-"true binary" mode, the memory consumption of xfce4-panel grows indefinitely.
It seems that this is due to xfce_clock_binary_draw_binary missing a g_date_time_unref (time); at the end.

I tested this on v4.14, but the bug seems to present all the way from 1e4d05c8 to current master.