GRIM: Set g_grim to nullptr in the deconstructor

If you start grim (or emi), have the md5 fail the verify. Tell it you do not want to play anyway, and then try again, it'll crash as g_grim is pointing to a non existent object.

I'm making a pull request because I want to make sure that this is not bad for some strange reason. I don't think there is any issue here, but I figure it's good to check.