server: break read_from_vdi_port loop if the guest gets disconnected
server: break read_from_vdi_port loop if the guest gets disconnected

read_from_vdi_port calls dispatch_vdi_port data, which will disconnect
the guest agent if it sends invalid data. It would then try to read more
data from the disconnected guest agent resulting in a NULL ptr dereference,
this patch fixes this.

SPICE/spice,fgouget/spice,alon/spice-server,freedesktop-unofficial-mirror/spice__spice,SPICE/spice,elmarco/spice,alon/spice-server,dezelin/spice,alon/spice-server,Fantu/spice,freedesktop-unofficial-mirror/spice__spice,mathslinux/spice,dezelin/spice,mathslinux/spice,ya-mouse/spice,mathslinux/spice,SPICE/spice,fgouget/spice,freedesktop-unofficial-mirror/spice__spice,fgouget/spice,alon/spice-server,fgouget/spice,freedesktop-unofficial-mirror/spice__spice,dezelin/spice,Fantu/spice,mathslinux/spice,Fantu/spice,elmarco/spice,mathslinux/spice,dezelin/spice,elmarco/spice,freedesktop-unofficial-mirror/spice__spice,alon/spice-server,Fantu/spice,ya-mouse/spice,elmarco/spice,dezelin/spice,ya-mouse/spice,elmarco/spice