[HarfBuzz] assert in hb_buffer_add_utf

[Dejan Kovacevic]

The hb_buffer_add_utf function starts with the following assert (hb-buffer.cc: line 1289):

  assert (buffer->content_type == HB_BUFFER_CONTENT_TYPE_UNICODE ||
      (!buffer->len && buffer->content_type == HB_BUFFER_CONTENT_TYPE_INVALID));

I have observed that buffer->content_type can also be HB_BUFFER_CONTENT_TYPE_GLYPHS, in which case the assert fails. Not sure if this is by design.