Update block.c

Correct a long standing bug - decrease the number of parent blocks when a set of blocks are recombined.

ref/block.c

@@ -381,6 +381,7 @@ void consolidate_blocks(void)
  bp->number = pp->number;
  bp->num_prime = pp->num_prime;
  pp->number = -1;
+ num_parents--;
  bp->level = pp->level;
  bp->parent = pp->parent;
  bp->parent_node = pp->parent_node;