CVE-2026-89963

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

powerpc/kexec_file: Fix null-ptr-def in extra size calculation

A static Sashiko AI review identified a potential NULL pointer
dereference in kexec_extra_fdt_size_ppc64().

On platforms without any reserved memory regions,
get_reserved_memory_ranges() can return 0 while leaving 'rmem'
unallocated as NULL. Passing it directly leads to a kernel panic when
evaluating 'rmem->nr_ranges'.

Add a NULL check for 'rmem' to prevent this crash.