ui: avoid crash if vnc client disconnects with writes pending

The vnc_client_read() function is called from the vnc_client_io()
event handler callback when there is incoming data to process.
If it detects that the client has disconnected, then it will
trigger cleanup and free'ing of the VncState client struct at
a safe time.

Unfortunately, the vnc_client_io() event handler will also call
vnc_client_write() to handle any outgoing data writes. So if
vnc_client_io() was invoked with both G_IO_IN and G_IO_OUT
events set, and the client disconnects, we may try to write to
a client which has just been freed.

https://bugs.launchpad.net/qemu/+bug/1594861

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-id: 1467042529-3372-1-git-send-email-berrange@redhat.com
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
