QRawFont: improve performance and safety of glyphIndexesForString()

QRawFont: improve performance and safety of glyphIndexesForString()

As of 98c1eb1750498cdff9d3b26658e5e5be9c026c92, partially initialized
QGlyphLayout is ok for stringToCMap() if GlyphIndicesOnly flag is set,
thus we can use the glyphIndexes buffer directly and avoid copying.
Also add some checks to guarantee we're not falling into an undefined behavior
for the empty text or NULL buffer.

Change-Id: I662953703e4c65edbebabbe4b753972417d963f3
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@digia.com>