channel-main: Call g_simple_async_result_is_valid first
channel-main: Call g_simple_async_result_is_valid first

Call g_simple_async_result_is_valid before using the result.

Signed-off-by: Hans de Goede <9fa1be1a5b5729e4c6b404f34c9ce49ff4882fd8@redhat.com>

elmarco/spice-gtk,SPICE/spice-gtk,guodong/spice-gtk,Fantu/spice-gtk,elmarco/spice-gtk,Fantu/spice-gtk,SPICE/spice-gtk,SPICE/spice-gtk,Fantu/spice-gtk,guodong/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,Fantu/spice-gtk,fgouget/spice-gtk,flexVDI/spice-gtk,flexVDI/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,elmarco/spice-gtk,guodong/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk,fgouget/spice-gtk,SPICE/spice-gtk,elmarco/spice-gtk,fgouget/spice-gtk,guodong/spice-gtk,freedesktop-unofficial-mirror/spice__spice-gtk