Charlie Lee directly answered someone asking about a 51% segwit theft
https://twitter.com/SatoshiLite/status/930539604897173504Charlie Lee [LTC] @SatoshiLite
"En réponse à @TheEscapening
That's fud. Miners cannot steal from Bitcoin or Litecoin SegWit addresses even with 100% hashpower. This is because users are now enforcing the softfork. Please educate and stop spreading misinformation."
He mentions ressources to read further. I am not technical enough to comment.