(WIP) Try to define a callback in another Lua state, then call it from the
original Lua state. Fails with the segmentation fault below.
$ export LANG=C
$ luajit thread2.lua
function: 0x40f3bba8
func_ptr        cdata<const void *>: 0x40f3bba8
thread  cdata<uint64_t [1]>: 0x415472e0
thread[0]       0ULL
A res   0
Segmentation fault