When enabling the sanitizer build it will complain about control
reaching a non-void function. Normally the compiler should detect that
there is only one possible exit given a static VNC_SERVER_FB_BYTES.

As we always expect a static VNC_SERVER_FB_BYTES I've added a compile
time assert and just called the sub-function directly.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>