Allow the pixel array to be NULL in restorescreen().

This allows modifying a savescreen() rather than needing to vmem_gettext()
stuff, but also change things without hacking up the pixels array yourself.

src/conio/ciolib.c

@@ -1883,7 +1883,7 @@ CIOLIBEXPORT int CIOLIBCALL ciolib_restorescreen(struct ciolib_screen *scrn)
 	ciolib_textcolor(scrn->text_info.attribute);
 	ciolib_window(scrn->text_info.winleft, scrn->text_info.wintop, scrn->text_info.winright, scrn->text_info.winbottom);
 	vmode = find_vmode(scrn->text_info.currmode);
-	if (vmode != -1)
+	if (vmode != -1 && scrn->pixels != NULL)
 		ciolib_setpixels(0, 0, vparams[vmode].charwidth * vparams[vmode].cols - 1, vparams[vmode].charheight * vparams[vmode].rows - 1, 0, 0, scrn->pixels, NULL);
 	for (i=0; i<5; i++)
 		ciolib_setfont(scrn->fonts[i], FALSE, i);