WeakReferenceable: Clear fUseCount when it is 1.

Otherwise, if there are remaining weak references, they could
potentially try to acquire the object and trigger a use-after-free.

headers/private/shared/WeakReferenceable.h

@@ -30,6 +30,8 @@ public:
 			void				GetUnchecked();
 
 private:
+			friend class BWeakReferenceable;
+
 			int32				fUseCount;
 			BWeakReferenceable*	fObject;
 };