fbdev/deferred-io: Test smem_start for I/O memory

Test smem_start before looking up pages from its value. Return
NULL if it is unset. This will result in a SIGBUS signal.
Signed-off-by: Thomas Zimmermann <tzimmermann@suse.de>
Reviewed-by: Javier Martinez Canillas <javierm@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20240419083331.7761-6-tzimmermann@suse.de

drivers/video/fbdev/core/fb_defio.c

@@ -26,11 +26,11 @@
 static struct page *fb_deferred_io_page(struct fb_info *info, unsigned long offs)
 {
 	const void *screen_buffer = info->screen_buffer;
-	struct page *page;
+	struct page *page = NULL;
 
 	if (is_vmalloc_addr(screen_buffer + offs))
 		page = vmalloc_to_page(screen_buffer + offs);
-	else
+	else if (info->fix.smem_start)
 		page = pfn_to_page((info->fix.smem_start + offs) >> PAGE_SHIFT);
 
 	return page;