mm: huge_memory: handle strsep not finding delimiter

split_huge_pages_write() does not handle the case where strsep finds no
delimiter in the given string and sets the input buffer to NULL, which
allows this reproducer to trigger a protection fault.

Link: https://lkml.kernel.org/r/20241216042752.257090-2-leocstone@gmail.com


Signed-off-by: Leo Stone <leocstone@gmail.com>
Reported-by:  <syzbot+8a3da2f1bbf59227c289@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=8a3da2f1bbf59227c289


Signed-off-by: Andrew Morton <akpm@linux-foundation.org>