Fix an issue with `WeakRef`s being over-released

As an optimisation, on load of a weak reference we check if the object
has already been deallocated and, if so, decrement the weak reference
count and zero the pointer to the weak reference structure so that the
next check is faster and doesn't need to hold locks for as long.

Unfortunately, the prior implementation of this instead decremented the
weak reference count and then only zeroed the pointer if the reference
count reached zero.  This meant that loading the same __weak pointer
twice after the pointed-to object had been deallocated would decrement
the reference count twice.

arc.m

@@ -856,8 +856,9 @@ OBJC_PUBLIC id objc_loadWeakRetained(id* addr)
 	if (obj == nil)
 	{
 		// If we've destroyed this weak ref, then make sure that we also deallocate the object.
-		if (weakRefRelease(ref))
+		if (ref != NULL)
 		{
+			weakRefRelease(ref);
 			*addr = nil;
 		}
 		return nil;