by avoiding the glyph metrics calculation when those metrics weren't requested
(QFontEngine::GlyphIndicesOnly flag has been passed).
As a side effect, this fixes a crash in case QGlyphLayout
was initialized only partially (with the glyphs array and the size).
Change-Id: I7d67abc2a74683131361fa21f8be203f61f247bc
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>