ui/vnc: Do not copy z_stream

ui/vnc: Do not copy z_stream

vnc_worker_thread_loop() copies z_stream stored in its local VncState to
the persistent VncState, and the copied one is freed with deflateEnd()
later. However, deflateEnd() refuses to operate with a copied z_stream
and returns Z_STREAM_ERROR, leaking the allocated memory.

Avoid copying the zlib state to fix the memory leak.

Fixes: bd023f953e5e ("vnc: threaded VNC server")
Signed-off-by: Akihiko Odaki <odaki@rsg.ci.i.u-tokyo.ac.jp>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Message-Id: <20250603-zlib-v3-1-20b857bd8d05@rsg.ci.i.u-tokyo.ac.jp>