Bugfix, autoreleased error reference

[jonkan]

Solves the crash when setting the successPtr/errorPtr in a MTLValueTransformer, e.g.

    @objc class func someJSONTransformer() -> ValueTransformer & MTLTransformerErrorHandling {
        return MTLValueTransformer(usingForwardBlock: { (value, successPtr, errorPtr) -> Any? in
            errorPtr?.pointee = NSError(domain: "some", code: 0, userInfo: nil)
            successPtr?.pointee = false
            return nil
        }, reverse: { (value, successPtr, errorPtr) -> Any? in
            errorPtr?.pointee = NSError(domain: "some", code: 0, userInfo: nil)
            successPtr?.pointee = false
            return nil
        })
    }