An Escondido physician was shot and fatally wounded by a patient who had come to his office complaining of stomach pains. Dr. Craig R. Blundell, 37, a gastroenterologist who had practiced in Escondido since 1980, was pronounced dead at Palomar Memorial Hospital. Police arrested Steve Alan Larson, 30, an unemployed engineer whom witnesses identified as having come to Blundell's office for examination about an hour before the slaying. Larson was booked on suspicion of murder but after several hours of questioning, police said they had been unable to establish a motive. They said Larson had never seen the doctor before coming to him as a patient.