Fixed a regression where spectators couldn't get back to their own view after spying on another player.

After fixing https://zandronum.com/tracker/view.php?id=3875 via 99fede56d6fa, spectators are unable to revert the view back to themselves if they spied on another player. This is because PLAYER_IsValidPlayerWithMo( pnum ) returns false if pnum is a spectator, which is the case when the spectator is changing the view to themselves. We should be more concerned that pnum and players[pnum].mo are valid.