"verify_level" command crashes game if run without level loaded

[GooberRF]

This definitely isn't something that would come up much (why did I even try this?), but the game does crash if you run verify_level in the main menu, for example. The command should ideally only be allowed to be run when a level is loaded.

The "No level loaded!" behaviour from the level_info command could just be replicated to this command.