The murder trial of an Easthampton man charged with killing his girlfriend has been postponed while the defendant undergoes an evaluation to determine whether he is competent enough to represent himself. Ryan Welch’s trial, scheduled to start Monday in Hampshire Superior Court, was delayed during a court hearing Wednesday. No new date was set. A judge said a court-appointed doctor would need ‘‘considerable additional time’’ to evaluate whether Welch is competent to act as his own lawyer. Welch, 38, maintains that he did not kill Jessica Ann Pripstein.