init polyobject thinker references to 0

otherwise its uncertain and very unlikely for them to be removed
P_RemoveThinker(Delayed) checks if thinkers are still referenced before removal.
polyobject thinkers never initialised the reference count but called thinker removal, this lead to checking unitialised values and those thinkers getting stuck in removal limbo without ever being actually removed

src/p_polyobj.c

@@ -150,6 +150,8 @@ FUNCINLINE static ATTRINLINE void PolyObj_AddThinker(thinker_t *th)
 	th->next = thinkercap.next;
 	th->prev = &thinkercap;
 	thinkercap.next = th;
+
+	th->references = 0; // init to 0 so they can be properly removed
 }
 
 //