Not sure why the code from f5e72070
did not produce correct results any longer. Anyway, now OpenCL
division works right in case of empty or zero cells.

Clearly I need to add unit tests to make sure this stuff keeps
working. In later commits.

Change-Id: I93b787ad5da453af1601768308fb614a332ed142
Signed-off-by: Michael Meeks <michael.meeks@collabora.com>
(cherry picked from commit 958ef1a5)