mm/hugetlb: fix early boot crash on parameters without '=' separator

If hugepages, hugepagesz, or default_hugepagesz are specified on the
kernel command line without the '=' separator, early parameter parsing
passes NULL to hugetlb_add_param(), which dereferences it in strlen() and
can crash the system during early boot.

Reject NULL values in hugetlb_add_param() and return -EINVAL instead.

Link: https://lore.kernel.org/20260409105437.108686-4-thorsten.blum@linux.dev
Fixes: 5b47c02967ab ("mm/hugetlb: convert cmdline parameters from setup to early")
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Reviewed-by: Muchun Song <muchun.song@linux.dev>
Cc: David Hildenbrand <david@kernel.org>
Cc: Frank van der Linden <fvdl@google.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>