fs/zfs: Check if returned pointer for allocated memory is NULL

fs/zfs: Check if returned pointer for allocated memory is NULL

When using grub_malloc() or grub_zalloc(), these functions can fail if
we are out of memory. After allocating memory we should check if these
functions returned NULL and handle this error if they did.

Signed-off-by: Lidong Chen <lidong.chen@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
Gbp-Pq: Topic cve-2025-jan
Gbp-Pq: Name fs-zfs-Check-if-returned-pointer-for-allocated-memory-is-.patch