agent: fix mishandling of agent data received from the client after agent disconnection
agent: fix mishandling of agent data received from the client after agent disconnection

The server can receive from the client agent data even when the agent
is disconnected. This can happen if the client sends the agent data
before it receives the AGENT_DISCONNECTED msg. We should receive and handle such msgs, instead
of disconnecting the client.
This bug can also lead to a server crash if the agent gets reconnected
fast enough, and it receives an agent data msg from the client before MSGC_AGENT_START.

upstream bz#55726
rhbz#881980

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