gtk: fix handling of incoming large clipboard data
gtk: fix handling of incoming large clipboard data

The main fix was probably the agent_msg_pos guin8 -> guint, although I
modified the code for more clarity

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