A rare segfault was observed when exonerate (2.2.0) was invoked with "--refine
region".  Refined alignments can (rarely) have a score less than the unrefined
alignment, so call Optimal_find_path() with a threshold of 0 so it doesn't
destroy the refined alignment if its score is less than that of the unrefined
alignment, and use the unrefined alignment if its score is > that of the
refined alignment.