fbdev: sm501fb: Add some geometry checks.

Added checks for xoffset, yoffset settings.
Incorrect settings of these parameters can lead to errors
in sm501fb_pan_ functions.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 5fc404e ("[PATCH] fb: SM501 framebuffer driver")
Signed-off-by: Danila Chernetsov <listdansp@mail.ru>
Signed-off-by: Helge Deller <deller@gmx.de>

Author: DanCh2020

drivers/video/fbdev/sm501fb.c

@@ -327,6 +327,13 @@ static int sm501fb_check_var(struct fb_var_screeninfo *var,
 	if (var->xres_virtual > 4096 || var->yres_virtual > 2048)
 		return -EINVAL;
 
+	/* geometry sanity checks */
+	if (var->xres + var->xoffset > var->xres_virtual)
+		return -EINVAL;
+
+	if (var->yres + var->yoffset > var->yres_virtual)
+		return -EINVAL;
+
 	/* can cope with 8,16 or 32bpp */
 
 	if (var->bits_per_pixel <= 8)