Investors were slamming the brakes on Fisker (FSR -33.74%) stock today. Shares of the electric vehicle (EV) start-up were tumbling after the company issued the dreaded "going concern" warning in a filing after hours yesterday. Essentially, that means the company is running out of cash and may not be able to stay in business.Fisker also reported preliminary fourth-quarter earnings and said it was in advanced talks to take an investment from a major automaker, which was later reported to be Nissan.As a result ...