Fix possible crash due to integer overflow

QFontEngineMulti::stringToCMap() stores the fallback engine index
in a glyph index'es high byte, which means the maximum fallback engine index
it can store is 255, so limit the number of tries we're doing to this value.
Otherwise we could end up with `fontEngineMulti->engine(glyph >> 24) == 0`
after successful stringToCMap() call.

Task-number: QTBUG-30412

Change-Id: I06907a39186fd207f3ce4b732a1a54e615744082
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
initCharAttributes() micro-optimization

Change-Id: Id8e275c9b4ae0a9855b8ba8917824c79cde5a919
Reviewed-by: Lars Knoll <lars.knoll@digia.com>