fbdev: savage: Error out if pixclock equals zero

In the Linux kernel, the following vulnerability has been resolved: fbdev: savage: 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-by-zero error. Although pixclock is checked in savagefb_decode_var(), but it is not checked properly in savagefb_probe(). Fix this by checking whether pixclock is zero in the function savagefb_check_var() before info->var.pixclock is used as the divisor. This is similar to CVE-2022-3061 in i740fb which was fixed by commit 15cf0b8.