Fix access of non-existing color table

Since alphaMapForGlyph now returns images in alpha8 format, they no
longer have a colortable, so we need to fix the fallback implementation
of alphaRGBMapForGlyph from trying to access it.

Task-number: QTBUG-41855
Change-Id: I232089163cfc817d7cf16df566f05629a968bf12
Reviewed-by: Ulf Hermann <ulf.hermann@digia.com>
Reviewed-by: Gunnar Sletta <gunnar@sletta.org>
Make QRingBuffer::ungetChar() not leave empty array in buffer list

Change-Id: Ic692693132f0c316ad068cad5423ca8fd32bd40d
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>