vnc: fix use-after-free

vnc: fix use-after-free

When vnc_client_read() return value is -1
vs is not valid any more.

Fixes: d49b87f0d1e0520443a990fc610d0f02bc63c556
Reported-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Tested-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Tested-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20180420084820.3873-1-kraxel@redhat.com