- If duration is unknown (-1), treat it as zero for the sum. The error
  of one microsecond per item was usually negligible. But it was
  catastrophic if the total length was actually zero: we would return a
  negative total.
- Do not special case input item node type. Other item types can have
  children, e.g. directories.
- Remove redundant negative test.

(This does not fix the unlikely signed integer overflow if the total
length exceeds INT64_MAX.)