Janesville police are investigating an early morning stabbing.  Police say a 21-year-old Janesville man turned up at the hospital emergency room about 3:30 a.m. Tuesday.    Authorities say technicians did gather evidence at a home in the 400 block of North Walnut Street and investigators have interviewed available witnesses.  The victim was treated and released.  Police do not believe the incident represents a threat to the public at large.