main: fix recent main_channel_reset() warning
main: fix recent main_channel_reset() warning

Fix wrong warning added in 0704147d.

(lt-spicy:17511):
GSpice-WARNING **: (channel-main.c:415):spice_main_channel_reset:
runtime check failed: (c->agent_connected == FALSE)

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