ClassExercises/Solution-4.scala lose one "if" logic

[yuxh]

   if(h < 0 || **(h==0 && m<0)**) {
        new SimpleTime(0, 0)
    }else if(m >= 0) {
      new SimpleTime(h, m)
    }
    else new SimpleTime(h-1 ,60+m)

test:

val st4 = new SimpleTime(9, 30).subtract(new SimpleTime(9, 50))
st4.hours is 0
st4.minutes is 0