video: fbdev: neofb: add a check against divide error

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 because of the 'PICOS2KHZ' macro.

Fix this by checking whether 'pixclock' is zero first.

The following log reveals it:

[   53.093806] divide error: 0000 [#1] PREEMPT SMP KASAN PTI
[   53.093838] CPU: 3 PID: 11763 Comm: hang Not tainted 5.14.0-rc2-00478-g2734d6c1b1a0 #215
[   53.093859] RIP: 0010:neofb_check_var+0x80/0xe50
[   53.093951] Call Trace:
[   53.093956]  ? neofb_setcolreg+0x2b0/0x2b0
[   53.093968]  fb_set_var+0x2e4/0xeb0
[   53.093977]  ? fb_blank+0x1a0/0x1a0
[   53.093984]  ? lock_acquire+0x1ef/0x530
[   53.093996]  ? lock_release+0x810/0x810
[   53.094005]  ? lock_is_held_type+0x100/0x140
[   53.094016]  ? ___might_sleep+0x1ee/0x2d0
[   53.094028]  ? __mutex_lock+0x620/0x1190
[   53.094036]  ? do_fb_ioctl+0x313/0x700
[   53.094044]  ? mutex_lock_io_nested+0xfa0/0xfa0
[   53.094051]  ? __this_cpu_preempt_check+0x1d/0x30
[   53.094060]  ? _raw_spin_unlock_irqrestore+0x46/0x60
[   53.094069]  ? lockdep_hardirqs_on+0x59/0x100
[   53.094076]  ? _raw_spin_unlock_irqrestore+0x46/0x60
[   53.094085]  ? trace_hardirqs_on+0x6a/0x1c0
[   53.094096]  do_fb_ioctl+0x31e/0x700

Signed-off-by: Zheyu Ma <zheyuma97@gmail.com>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Link: https://patchwork.freedesktop.org/patch/msgid/1626871424-27708-1-git-send-email-zheyuma97@gmail.com