Interaction between cffi and lgi causes crashes

Hi, please see the below code for demo.

```lua
-- Uncommenting the line below will introduce a crash in callback:free().
require("cffi")
local lgi = require("lgi")
local callback = require("cffi").cast("void (*)()", function() end)
print("Before free")
callback:free()
print("After free")
```

I'm using cffi from luarocks, lua 5.1, and lgi from Debian repo (unstable).
