sort: fix problems with merge node dest pointer

sort: fix problems with merge node dest pointer

* src/sort.c (mergelines_node): Return void, not size_t.  All
callers changed.  Change *node->dest here, not in caller.
Do not change node->dest: it's not needed and could cause problems
on (mostly theoretical) hosts that do not allow adding integers to
null pointers.
(queue_check_insert_parent): Omit MERGED parameter; no longer needed.
All callers changed.