per_commitment_secret must be a valid secret key

See #832

Author: LLFourn

02-peer-protocol.md

@@ -1080,7 +1080,7 @@ A sending node:
   transaction.
 
 A receiving node:
-  - if `per_commitment_secret` does not generate the previous `per_commitment_point`:
+  - if `per_commitment_secret` is not a valid secret key or does not generate the previous `per_commitment_point`:
     - MUST fail the channel.
   - if the `per_commitment_secret` was not generated by the protocol in [BOLT #3](03-transactions.md#per-commitment-secret-requirements):
     - MAY fail the channel.