Maybe. People die in their sleep all the time. Who knows...might have died in their dreams and the fear associated caused them to have a heart attack. If it happens, you won't know it. Don't stress over what may or may not happen. You are gonna die when you're gonna die...no matter what you're doing at the time. You can't change that. Don't worry yourself into a panic or a psych hospital.
in some cases that may be true but probably not like your thinking there is a Syndrome" or "SUNDS" for short; this is now theorized to be a form of Brugada syndrome. this is mainly caused by symptoms of sleep paralysis, where the body freezes when awaking from a dream, causing shock leading to possible heart attack
Never. Dreams are just images and sensations occurring during sleep. I'd however like to cite a coincidence. Actor David Janssen dreamed that he was being carried in a coffin following a heart attack two days prior to his heart attack (something that he told his wife) that resulted in his death.