BUILD: acme: fix false positive null pointer dereference

BUILD: acme: fix false positive null pointer dereference

src/acme.c: In function ‘cfg_parse_acme_vars_provider’:
src/acme.c:471:9: error: potential null pointer dereference [-Werror=null-dereference]
  471 |         free(*dst);
      |         ^~~~~~~~~~

gcc13 on ubuntu 24.04 detects a false positive when building
3e72a9f ("MINOR: acme: provider-name for dpapi sink").
Indeed dst can't be NULL. Clarify the code so gcc don't complain
anymore.