If and when a player leaves a multiplayer game, everything owned by that player leaves the game with him/her and no longer has any effect on the game.
800.4a When a player leaves the game, all objects (see rule 109) owned by that player leave the game and any effects which give that player control of any objects or players end. Then, if that player controlled any objects on the stack not represented by cards, those objects cease to exist. Then, if there are any objects still controlled by that player, those objects are exiled. This is not a state-based action. It happens as soon as the player leaves the game. If the player who left the game had priority at the time he or she left, priority passes to the next player in turn order who’s still in the game.
If a creature doesn't make it to the combat damage step, it won't deal combat damage.
506.1. The combat phase has five steps, which proceed in order: beginning of combat, declare attackers, declare blockers, combat damage, and end of combat.
Also note that combat damage technically doesn't "resolve" because it doesn't go on the stack (it used to), all creatures deal their damage simultaneously in their respective damage steps.