lua: don't destroy keys during table iteration

[Habbie]

via ahupowerdns/luawrapper#38

Short description

This avoids a dnsdist crash when code returns an array-shaped table (i.e. return {1,2,3}).

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)
• checked that this code was merged to master

[Habbie]

@cmouse thanks, your changes make sense - I'll upstream those first to go through the LuaWrapper testsuite.

[omoerbeek]

An alternative I like a little better: (it has two pop calls which isn't very nice either):
But approved either way.

if (val != nullptr) {
  std::string result;
  result.assign(val, len);
  lua_pop(state, 1);
  return boost::optional<std::string>{ std::move(result) };
}
else {
  lua_pop(state, 1);
  return boost::none;
}