The murder trial of Upper Gwynedd veterinarian David Rapoport has been postponed until early next year.   It had been scheduled to start a week from Monday.   He faces two homocide charges for the deaths of his girlfriend and unborn child.   She worked with Rapoport at the Harleysville Veterinary Hospital and was shot several times last March.   Her body was found in the woods near Allentown.   Rapoport remains in Lehigh County Prison without any possibility of bail and could get the death penalty if convicted.