The Hangul module produces hexboxes with all zeros in it in a couple of places:

One:

  /* Find a base character to render the mark on
   */
  int index = find_char (font, 0x25cc); /* DOTTED CIRCLE */
(pango_font_describe (font)));
  if (!index)
    index = find_char (font, 0x25cb);   /* WHITE CIRCLE, in KSC-5601 */
  if (!index)
    index = find_char (font, ' ');      /* Space */
  if (!index)                           /* Unknown glyph box with 0000 in it */
    index = find_char (font, get_unknown_glyph (font, 0));

This doesn't look that bad.

The other:

          index = (wc >= 0x3131) ? find_char (font, wc) : 0;
          pango_glyph_string_set_size (glyphs, *n_glyphs + 1);
          if (!index)
            set_glyph (font, glyphs, *n_glyphs, cluster_offset,
                       get_unknown_glyph (font, index));
          else
            set_glyph (font, glyphs, *n_glyphs, cluster_offset, index);

The first set_glyph is definitely wrong.  I'm changing it to:

          index = (wc >= 0x3131) ? find_char (font, wc) : 0;
          pango_glyph_string_set_size (glyphs, *n_glyphs + 1);
          if (!index)
            set_glyph (font, glyphs, *n_glyphs, cluster_offset,
                       get_unknown_glyph (font, text[i]));
          else
            set_glyph (font, glyphs, *n_glyphs, cluster_offset, index);