char-device: set to NULL freed pointers on destroy
char-device: set to NULL freed pointers on destroy

As SpiceCharDeviceState is only unref'ed on
spice_char_device_state_destroy the same device could be destroyed more
then once so the pointers that are freed should be set to NULL.

Related: https://bugzilla.redhat.com/show_bug.cgi?id=1281455

fgouget/spice,fgouget/spice,fgouget/spice,fgouget/spice,SPICE/spice,SPICE/spice,SPICE/spice