elena curtin was seven-months pregnant during portland , oregon , incident .
boyfriend 's ex was injecting drugs in her bathroom when she came home .
curtin , 23 , was scheduled to go to trial on second-degree assault charge .
she hit boyfriend 's ex on head and arm but prosecutors dropped charge .
oregon law allows for physical force against an intruder who wo n't leave .
conviction would have resulted in mandatory six-year prison sentence .