Skip sub-group.  Returns the new position.

def _SkipGroup(buffer, pos, end):
  """Skip sub-group.  Returns the new position."""

  while 1:
    (tag_bytes, pos) = ReadTag(buffer, pos)
    new_pos = SkipField(buffer, pos, end, tag_bytes)
    if new_pos == -1:
      return pos
    pos = new_pos