fix threaded conversion with linearisation curves optimisation

The first commit fixes following bug, while the second commit is an optimisation to the bug fix.

Bug: Using threads and cmsFLAGS_CLUT_XXX_LINEARIZATION
Above scenario causes unexpected scattered colours.
The effect can be easily observed by using a threaded conversion and enabling
the cmsFLAGS_CLUT_POST_LINEARIZATION and cmsFLAGS_CLUT_PRE_LINEARIZATION
flags. This was seen in Oyranos' image_display viewer, which uses lcms and
OpenMP threads.

The changes fix the above observed bug.