FSAA sample files in EXR format are no longer always updated (after some
changes between 2.73 and 2.74 releases), and the reported bug was caused
by old samples from a previous frame that were being merged by mistake.
The present revision addresses the documented issue by entirely skipping
the rendering of auto-generated scenes when there are no Freestyle strokes
to render, which suppresses sample merging of the render layers that were
not rendered.
c8e9832be3