worker: do not leak cursor after disconnecting clients
worker: do not leak cursor after disconnecting clients

Calling cursor_channel_disconnect does not free cursor_display
so this causes a leak.
Is the only code where this pointer is reset preventing any
further cursor channel connection. If a client is lazy reading
cursor data during the flush connection is closed and further clients
won't be able to use the cursor.
This also prevents future use of a NULL pointer.

Signed-off-by: Frediano Ziglio <55d48b080b2e443e395cde84d2c83b135a4ff48e@redhat.com>
Acked-by: Jonathon Jongsma <jjongsma at redhat.com>

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