BUG/MINOR: hlua: fix reference leak in hlua_post_init_state()

BUG/MINOR: hlua: fix reference leak in hlua_post_init_state()

hlua init function references were not released during
hlua_post_init_state().

Hopefully, this function is only used during startup so the resulting
leak is not a big deal.
Since each init lua function runs precisely once, it is safe to release
the ref as soon as the function is restored on the stack.

This could be backported to every stable versions.
Please note that this commit depends on "MINOR: hlua: add simple hlua reference handling API"