Fix crashes from incomplete resets
Fix crashes from incomplete resets

`side.lastPokemon` could retain a destroyed Pokemon, leading to
crashes.

Zarel/Pokemon-Showdown-Client,sirDonovan/Pokemon-Showdown-Client,sirDonovan/Pokemon-Showdown-Client,Zarel/Pokemon-Showdown-Client,sirDonovan/Pokemon-Showdown-Client,sirDonovan/Pokemon-Showdown-Client,Zarel/Pokemon-Showdown-Client,Zarel/Pokemon-Showdown-Client