fbdev: udlfb: avoid divide-by-zero on FBIOPUT_VSCREENINFO

Much like commit 19f953e ("fbdev: fb_pm2fb: Avoid potential divide
by zero error"), we also need to prevent that same crash from happening
in the udlfb driver as it uses pixclock directly when dividing, which
will crash.

Cc: Bernie Thompson <[email protected]>
Cc: Helge Deller <[email protected]>
Fixes: 59277b6 ("Staging: udlfb: add dynamic modeset support")
Assisted-by: gregkh_clanker_t1000
Signed-off-by: Greg Kroah-Hartman <[email protected]>
Signed-off-by: Helge Deller <[email protected]>

Author: gregkh

drivers/video/fbdev/udlfb.c

@@ -1018,6 +1018,9 @@ static int dlfb_ops_check_var(struct fb_var_screeninfo *var,
 	struct fb_videomode mode;
 	struct dlfb_data *dlfb = info->par;
 
+	if (!var->pixclock)
+		return -EINVAL;
+
 	/* set device-specific elements of var unrelated to mode */
 	dlfb_var_color_format(var);