cfg: Do not clear the global configuration if there was no old config

If we had no old configuration, then in cfg_run_parse(), do not re-set
the configuration global variable to old_cfg, because then it will be
NULL, and things that happen to want this variable past that point
will not work correctly.

Reported-by: Balint Kovacs <blint@balabit.hu>
Signed-off-by: Gergely Nagy <algernon@balabit.hu>