Fix ages old segfault on trying to expand %{lua} (RhBug:972646)

Fix ages old segfault on trying to expand %{lua} (RhBug:972646)

- Using the already calculated macro arg + arglen for copying the
  buffer instead of buggy local variant helps... This is as old as
  the embedded lua interpreter in rpm, close to a decade.
- Besides fixing the segfault, this actually makes it behave like
  other built-ins, evaluating to empty string when an empty arguments
  or no arguments are passed.
(cherry picked from commit ae5795897159319923b60f5c141a2ae5aa6f8d68)