[ldap] fix error handling when no available conn

Do not try to release/free a conn if we do not have one

(cherry picked from commit 27acf2a)

modules/ldap/ldap_exp_fn.c

@@ -159,7 +159,8 @@ int ldap_search_impl_async(
  return 1;
 
 error:
- release_ldap_connection(conn);
+ if (conn)
+ release_ldap_connection(conn);
  return rc;
 }