gdetect_write block write skipping bug

I ran into a small bug for the case when you disable learning for a block after you've already learned some parameters for it.  gdetect_write sometimes fails to write these blocks.

The scores computed by the fv_cache will not match the scores computed in the poslatent step since some blocks will be incorrectly 0 in the cache.  When the hinge loss is checked, the fv_cache scores might be higher than the relabeled positives and so the outer relabeling loop will terminate.

Your comment is correct, but the code doesn't match.