agent: don't attempt to read from the device if it was released
agent: don't attempt to read from the device if it was released

if vdi_port_read_buf_process failes, we detach the agent and also release
the read buffer. We shouldn't try reading from the device afterwards.

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