Spawn spots not preserved in saved games

Doom automatically generates a list of all Boss Brain spawn spots at the beginning of each map, but it neglects to do this again when loading a saved game. This means that any Boss Brains on the map will become inoperative. In addition, if there are any Boss Brain 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 monster targets are not saved in savegames. The SpawnFly codepointer neglects to check that the cube's target is valid before using it, causing it to access a NULL pointer.