video: fbdev: kyro: Error out if 'lineclock' equals zero

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

Fix this by checking whether 'lineclock' is zero.

The following log reveals it:
 divide error: 0000 [#1] PREEMPT SMP KASAN PTI
 RIP: 0010:kyrofb_set_par+0x30d/0xd80
 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: Zheyu Ma <zheyuma97@gmail.com>
Signed-off-by: Helge Deller <deller@gmx.de>

drivers/video/fbdev/kyro/fbdev.c

@@ -494,6 +494,8 @@ static int kyrofb_set_par(struct fb_info *info)
 				    info->var.hsync_len +
 				    info->var.left_margin)) / 1000;
 
+	if (!lineclock)
+		return -EINVAL;
 
 	/* time for a frame in ns (precision in 32bpp) */
 	frameclock = lineclock * (info->var.yres +