QFontEngineBox: Fix assertion for code points beyond BMP

...due to a typo introduced in d8225fab8fb8b21fb5d and stricter checks
introduced in 4e319ca4c47a99295c1.
This uses recently added QStringIterator to decrease the code duplication.

Change-Id: Ib1c2519db1c4d65f3ac14e6f9ae3fa272860b1dd
Reviewed-by: Simon Hausmann <simon.hausmann@digia.com>