Fail CRTC configuration if !vtSema

Unless we check for vtSema before calling into the CRTC and output callbacks,
we may end up trying to access video memory that no longer exists, leading to a
crash.  So if we don't have vtSema, return FALSE to the caller, indicating that
we didn't do anything.

Fixes #14444.

hw/xfree86/modes/xf86RandR12.c

@@ -743,6 +743,9 @@ xf86RandR12CrtcSet (ScreenPtr	pScreen,
     xf86CrtcPtr		*save_crtcs;
     Bool		save_enabled = crtc->enabled;
 
+    if (!crtc->scrn->vtSema)
+	return FALSE;
+
     save_crtcs = xalloc(config->num_output * sizeof (xf86CrtcPtr));
     if ((randr_mode != NULL) != crtc->enabled)
 	changed = TRUE;