elua: prepare for compatibility with cffi-lua (use ffi.nullptr)

elua: prepare for compatibility with cffi-lua (use ffi.nullptr)

This is necessary because the LuaJIT FFI implements the null
pointer to nil equality behavior, which is not possible to
do in standard Lua (as equality metamethod needs identical types
to trigger).

So replace it with ffi.nullptr as cffi-lua implements; in luajit
ffi.nullptr will just become nil and it'll work like before.