pstore/zone: Add a null pointer check to the psz_kmsg_read

kasprintf() returns a pointer to dynamically allocated memory
which can be NULL upon failure. Ensure the allocation was successful
by checking the pointer validity.
Signed-off-by: Kunwu Chan <chentao@kylinos.cn>
Link: https://lore.kernel.org/r/20240118100206.213928-1-chentao@kylinos.cnSigned-off-by: Kees Cook <keescook@chromium.org>

fs/pstore/zone.c

@@ -973,6 +973,8 @@ static ssize_t psz_kmsg_read(struct pstore_zone *zone,
 		char *buf = kasprintf(GFP_KERNEL, "%s: Total %d times\n",
 				      kmsg_dump_reason_str(record->reason),
 				      record->count);
+		if (!buf)
+			return -ENOMEM;
 		hlen = strlen(buf);
 		record->buf = krealloc(buf, hlen + size, GFP_KERNEL);
 		if (!record->buf) {