from
https://gist.github.com/sipa/89076911. Inherent ECDSA signature malleability
2. Non-DER encoded ECDSA signatures
3.-8. [Txin script altering stuff]
Non-malled:
1.) Rule: lower half S is better
(does the standard client or any other client produce differing signatures?)
2.) Rule: only DER, handled as of 0.8+
3.-8.) Rule: shorter tx_in script is better
edit: How to find malled tx in the first place? Use NTXID:
https://github.com/bitcoin/bitcoin/pull/3656