fix return value bug causing crash with blocks in retainBlock

I found the crash I mentioned in https://github.com/tapwork/HeapInspector-for-iOS/issues/2 is related to this bug.

It should return the address of copied object, via: http://clang.llvm.org/docs/AutomaticReferenceCounting.html

And according to the doc, if the object is no longer on stack (actually I didn't quite understand the meaning of on stack, retainCount > 0?), it should be retained instead of copied.

But with this fix, at least blocks won't crash with HeapInspector enabled.