fix(logic): Add nullptr check to thisPlayer in GameLogic::logicMessageDispatcher

[Caball009]

This PR changes the nullptr assertion on thisPlayer in GameLogic::logicMessageDispatcher to an early return. Although there are a couple of places in this function that check explicitly for it, a lot of code assumes that this pointer is not null, so it makes sense to check that up front.

Commit 1 adds the check.
Commit 2 removes code that's become redundant (implicitly).
Commit 3 changes msg->getPlayerIndex to thisPlayer->getPlayerIndex for the sole purpose of consistency with code that already uses the latter.

TODO:

• Replicate in Generals.

[Mauller]

on 645 we have

case GameMessage::MSG_ENABLE_RETALIATION_MODE:
{
	//Logically turns on or off retaliation mode for a specified player.
	Int playerIndex = msg->getArgument( 0 )->integer;
	Bool enableRetaliation = msg->getArgument( 1 )->boolean;

	Player *player = ThePlayerList->getNthPlayer( playerIndex );
	if( player )
	{
		player->setLogicalRetaliationModeEnabled( enableRetaliation );
	}
	break;
}

I wonder if this needs clarifying that player here is a targetPlayer and not the same as thisPlayer

I am guessing that Int playerIndex = msg->getArgument( 0 )->integer; returns a different value compared to msg->getPlayerID