Return a malformed HTLC message when ephemeral pubkey is garbage

Return a malformed HTLC message when ephemeral pubkey is garbage

This resolves a spec-compliance bug with BOLT 4 where we simply
failed to deserialize the message and thus could never return an
HTLC failure message. However, note that BOLT 4 incorrectly hints
that a non-malformed message should be used ("...MUST report a
route failure to the origin node") which we cannot do as we cannot
derive a SharedSecret to encrypt a regular update_fail_htlc message