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>

Author: leocstone

mm/huge_memory.c

@@ -4169,7 +4169,7 @@ static ssize_t split_huge_pages_write(struct file *file, const char __user *buf,
 		size_t input_len = strlen(input_buf);
 
 		tok = strsep(&buf, ",");
-		if (tok) {
+		if (tok && buf) {
 			strscpy(file_path, tok);
 		} else {
 			ret = -EINVAL;