[HarfBuzz] Error when using icu_le shaper

[Olivier Berten]

When trying to shape some khmer string with the option
--shapers=icu_le, hb-view crashes with

**
ERROR:helper-cairo.cc:467:void
helper_cairo_line_from_buffer(helper_cairo_line_t*, hb_buffer_t*,
const char*, unsigned int, double, hb_bool_t): assertion failed:
(hb_glyph[i].cluster > hb_glyph[i-1].cluster)


Olivier