Spawn spots not preserved in saved games

Doom automatically generates a list of all spawn spots for any final bosses present at the beginning of each map, but it neglects to do this again when loading a saved game. This means that any such bosses on the map will become inoperative. In addition, if there are any spawn cubes currently in flight, the game will crash due to the fact that the cube's target field is used to track the spot at which it will spawn a monster, and targets are not recorded in saved games. The SpawnFly codepointer neglects to check that the cube's target is valid before using it, causing it to access a NULL pointer.