Fix compiler complaints 'unused variable' used in assertion

simulate_mode.c: In function ‘ManifestStatInfo’:
simulate_mode.c:136:12: warning: variable ‘ret’ set but not used [-Wunused-but-set-variable]
  136 |     size_t ret = strftime(buf, sizeof(buf), "%Y-%m-%d %H:%M:%S %z",
      |            ^~~

mod_custom.c: In function ‘PromiseModule_SendMessage’:
mod_custom.c:365:22: warning: unused variable ‘line_length’ [-Wunused-variable]
  365 |         const size_t line_length = strlen(line);
      |                      ^~~~~~~~~~~

unix_iface.c: In function ‘FindV6InterfacesInfo’:
unix_iface.c:716:30: warning: unused variable ‘dst_length’ [-Wunused-variable]
  716 |                 const size_t dst_length = src_length - 1;
      |                              ^~~~~~~~~