Freeciv

I tend to startup the server and client at the same time. I think `real_menus_init()` returns because the ruleset has not yet been loaded, causing the `multiplayer_menu` variable to never become initialized.

Reduced, but not perfect, risk.

Perfectly remove the SIGSEGV, at the sacrifice of the menus. The menus are constantly rebuilt/updated, so eventually, the `real_menus_init()` should succeed.

Before fix
`multiplayer_menu` is NULL in `real_menus_update()` if `real_menus_init()` exits because the server ruleset is not ready.