Fix crash when connecting to VNC through websocket

Connecting to VNC through websocket crashes in vnc_flush() when trying
to acquire a mutex that hasn't been initialized (vnc_init_state(vs)
hasn't been called at this point).

Signed-off-by: Jorge Acereda Macia <jacereda@gmail.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
