QString::normalize(): Fix assertion in some corner case

Don't assume `from` is 0 and the string always starts with a starter code point.

This has been fixed for 5.0 as part of Unicode Data & Algorithms update
(qtbase:46b78113b22428e6f8540193fcf0e00591dbd724).

Task-number: QTBUG-30931

Change-Id: I2030aaf831ebe619b980e55e98d5f5a366dbe3ed
Reviewed-by: Lars Knoll <lars.knoll@digia.com>