video: fbdev: vt8623fb: Error out if 'pixclock' equals zero

The userspace program could pass any values to the driver through
ioctl() interface. If the driver doesn't check the value of 'pixclock',
it may cause divide error.

Fix this by checking whether 'pixclock' is zero in the function
vt8623fb_check_var().

The following log reveals it:
 divide error: 0000 [#1] PREEMPT SMP KASAN PTI
 RIP: 0010:vt8623fb_set_par+0xecd/0x2210
 Call Trace:
  <TASK>
  fb_set_var+0x604/0xeb0
  do_fb_ioctl+0x234/0x670
  fb_ioctl+0xdd/0x130
  do_syscall_64+0x3b/0x90
Signed-off-by: NZheyu Ma <zheyuma97@gmail.com>
Signed-off-by: NHelge Deller <deller@gmx.de>

drivers/video/fbdev/vt8623fb.c

@@ -321,6 +321,9 @@ static int vt8623fb_check_var(struct fb_var_screeninfo *var, struct fb_info *inf
 {
 	int rv, mem, step;
 
+	if (!var->pixclock)
+		return -EINVAL;
+
 	/* Find appropriate format */
 	rv = svga_match_format (vt8623fb_formats, var, NULL);
 	if (rv < 0)