Don't assert when HB-old is unable to deal with surrogates

Since we have no interest in fixing HB-old to properly support surrogates,
here is a simple workaround for the case when surrogate parts gets
treated like two separate glyphs in log_clusters but a single character.
And since it's a known issue that HB-old doesn't really support surrogates,
it's better to draw some garbage/boxes rather than to assert, IMO.

Task-number: QTBUG-39875
Change-Id: I95ff43e821ae989d7a6f792f71f157dcea92dd16
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>