Fix polyobject thinker leak

Before the introduction of seperate thinkerlists in srb2, polyobjects used their own way of adding thinkers to the list, however their "reference" value never gets initiliased. P_RemoveThinker(Delayed) checks if thinkers are still referenced before removal. This means on thinker removal it checks against an unitialised reference value and most of the time does not remove those thinkers This just initialises their reference to 0, so they can be always removed if theyre supposed to