video: fbdev: arkfb: 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.

The following log reveals it:
divide error: 0000 [#1] PREEMPT SMP KASAN PTI
RIP: 0010:arkfb_set_par+0x10fc/0x24f0
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 <[e-mail]>
Signed-off-by: Helge Deller <[e-mail]>

drivers/video/fbdev/arkfb.c

···
 {
 	int rv, mem, step;
 
+	if (!var->pixclock)
+		return -EINVAL;
+
 	/* Find appropriate format */
 	rv = svga_match_format (arkfb_formats, var, NULL);
 	if (rv < 0)