pstore/ram: Add kmsg hlen zero check to ramoops_pstore_write()

If zero-length header happened in ramoops_write_kmsg_hdr(), that means
we will not be able to read back dmesg record later, since it will be
treated as invalid header in ramoops_pstore_read(). So we should not
execute the following code but return the error.

Signed-off-by: Yue Hu <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>

fs/pstore/ram.c

···
 
 	/* Build header and append record contents. */
 	hlen = ramoops_write_kmsg_hdr(prz, record);
+	if (!hlen)
+		return -ENOMEM;
+
 	size = record->size;
 	if (size + hlen > prz->buffer_size)
 		size = prz->buffer_size - hlen;