Fix possible assertion when using stringToCMap() for a multi engine

Fix possible assertion when using stringToCMap() for a multi engine

If (*nglyphs < len), stringToCMap() sets *nglyphs to len and returns
false immediately; the caller then must resize the buffer and re-try.
However, QFontEngineMulti::stringToCMap() doesn't update the nglyphs value
and thus the second call would fail, too. This is quite unexpected.

Change-Id: Id2cce7b9faf7706c382fccf023e1b7affa9a10be
Reviewed-by: Lars Knoll <lars.knoll@digia.com>
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>