Simplified get_transfers and fixed sort segfault

[jagerman]

Most of this is simplification while I stepped through the function:

• use a for (x : list) loop instead of manual iterator handling
• don't bother double-checking args.in etc.: if they aren't specified the associated container will be empty anyway.
• replace (!((args.failed && is_failed) || (!is_failed && args.pending))) with logically equivalent !(is_failed ? args.failed : args.pending) (and then removed the negation, too).

The segfault is fixed by the change inside the sort: because only the case a.confirmed && !b.confirmed was handled but not the complement !a.confirmed && b.confirmed you could get a case where two transfer_views both compare earlier than each other.