Plus the first time we meet Lando, Han makes a reference to some incident where Han betrayed(?) Lando in the past...

"That was a long time ago. I'm sure he's forgotten all about that."

Lando forgives Han without a word (Just a gentle joshing) so I expect Han would forgive Lando too. It's that kind of friendship.

More importantly, we the audience had forgiven Lando before the credits rolled on ESB. So bringing it up again during the opening act of ROTJ a couple of yers later would be dramatically pointless.