BUG/MINOR: hlua: prevent crash when loading numerous arguments using lua-load(per...

BUG/MINOR: hlua: prevent crash when loading numerous arguments using lua-load(per-thread)

When providing multiple optional arguments with lua-load or
lua-load-per-thread directives, arguments where pushed 1 by 1
to the stack using lua_pushstring() without checking if the stack
could handle it.

This could easily lead to program crash when providing too much
arguments. I can easily reproduce the crash starting from ~50 arguments.

Calling lua_checkstack() before pushing to the stack fixes the crash:
  According to lua.org, lua_checkstack() does some housekeeping and
  allow the stack to be expanded as long as some memory is available
  and the hard limit isn't reached.
  When no memory is available to expand the stack or the limit is reached,
  lua_checkstacks returns an error: in this case we force hlua_load_state()
  to return a meaningfull error instead of crashing.
  In practice though, cfgparse complains about too many words
  way before such event may occur on a normal system.

  TLDR: the ~50 arguments limitation is not an issue anymore.

No backport needed, except if 'MINOR: hlua: Allow argument on
lua-lod(-per-thread) directives' (ae6b568) is backported.