channel: check if channel has a session
channel: check if channel has a session

Since 8943d2329, the channel may be disconnected from the session
before it's destroyed. In this case, session is NULL.

Fixes some critical with virt-manager when closing a display:

(virt-manager:20451): GSpice-CRITICAL **: spice_session_is_for_migration: assertion 'SPICE_IS_SESSION(session)' failed

Signed-off-by: Marc-André Lureau <a6f2b960a4aca61381fc650a67901b2ac8e83e6d@gmail.com>
Acked-by: Pavel Grunt <fbda40b445316123f12a5a6bd7556918ea74f4bc@redhat.com>

flexVDI/spice-gtk,flexVDI/spice-gtk,fgouget/spice-gtk,fgouget/spice-gtk,fgouget/spice-gtk