video: fbdev: cirrusfb: check pixclock to avoid divide by zero

In the Linux kernel, the following vulnerability has been resolved:

video: fbdev: cirrusfb: check pixclock to avoid divide by zero

Do a sanity check on pixclock value to avoid divide by zero.

If the pixclock value is zero, the cirrusfb driver will round up

pixclock to get the derived frequency as close to maxclock as

possible.

Syzkaller reported a divide error in cirrusfb_check_pixclock.

divide error: 0000 [#1] SMP KASAN PTI

CPU: 0 PID: 14938 Comm: cirrusfb_test Not tainted 5.15.0-rc6 #1

Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.0-2

RIP: 0010:cirrusfb_check_var+0x6f1/0x1260

Call Trace:

fb_set_var+0x398/0xf90

do_fb_ioctl+0x4b8/0x6f0

fb_ioctl+0xeb/0x130

__x64_sys_ioctl+0x19d/0x220

do_syscall_64+0x3a/0x80

entry_SYSCALL_64_after_hwframe+0x44/0xae