kern/efi/mm: Fix use-after-free in finish boot services

kern/efi/mm: Fix use-after-free in finish boot services

In grub-core/kern/efi/mm.c, grub_efi_finish_boot_services() has an instance
where the memory for the variable finish_mmap_buf is freed, but on the next
iteration of a while loop, grub_efi_get_memory_map() uses finish_mmap_buf. To
prevent this, we can set finish_mmap_buf to NULL after the free.

Signed-off-by: Alec Brown <alec.r.brown@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>