message: fix ElementLocationIterator.advance
  extra element was added if last unit size === 1
diff: @@ -956,6 +956,9 @@ namespace Structure {
  this.current.unit = this.structure.units[this.unitIndex];
  this.elements = this.current.unit.elements;
  this.maxIdx = this.elements.length - 1;
  + if (this.maxIdx === 0) {
  + this.hasNext = this.unitIndex + 1 < this.structure.units.length;
  + }
  }
  constructor(private structure: Structure) {
  