The previous code had some issues:

- race: priv_call->priv could be incomplete when being non-null, so
  using it outside re_mutex was not safe.

  fix: make sure that the re_t is complete before referencing it in
  priv_call->priv using a membar

- caching dynamic REs did not work because invalid comparison of
  the cached pattern, also freeing the cached REs was incorrect

  it could be a good idea to add a global cache for compiled
  patterns (dynamic or not), but as long as we don't have that,
  it's probably safest to just recompile dynamic REs always