blendBitmap24 assumes bitmap is 2 lines high for doing
interpolation. For bitmaps of height 1, really nothing to interpolate
there. Fix cornercase by 'blending' between the very same line instead.

Reviewed-on: https://gerrit.libreoffice.org/19863Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Norbert Thiebaud <nthiebaud@gmail.com>

Cherry-picked from 99e3ab6e

Change-Id: I9b94000aa563e525d0bb2418346ad2c86af26df8
Reviewed-on: https://gerrit.libreoffice.org/19888Reviewed-by: Katarina Behrens <Katarina.Behrens@cib.de>
Tested-by: Katarina Behrens <Katarina.Behrens@cib.de>
(cherry picked from commit 5a86268e)