Fix prunning in (sparse*sparse).pruned() when the result is nearly dense.
Fix prunning in (sparse*sparse).pruned() when the result is nearly dense.

ritsu1228/eigen,ritsu1228/eigen,pasuka/eigen,pasuka/eigen,pasuka/eigen,ritsu1228/eigen,pasuka/eigen,ritsu1228/eigen,ritsu1228/eigen,pasuka/eigen