Segfault in Lua exception handler

[François Perrad]

I try to restore the Lua behavior after ppd27mmd merge.

Lua crashes with segmentation fault when the Lua exception handler is
called (see subroutine 'docall' in languages/lua/src/lib/luaaux.pir),
when function NCI 'traceback' in languages/lua/src/pmc/lua.pmc is called.

The following commands reproduce the problem :

    $ cat add.lua
        print(true + 42)  --> attempt to perform arithmetic on a boolean
    $ parrot lua.pbc add.lua

or
    $ parrot luap.pir --target=pir add.lua > add.pir
    $ parrot add.pir



François.