Minor improvements of Chain::BlockPtrPtr optimization:
Minor improvements of Chain::BlockPtrPtr optimization:

- Fix typo in operator- which resulted in a missed optimization.

- Change repr from intptr_t to uintptr_t, so that operator< etc. can use
  the same code for is_special() and !is_special() cases.

PiperOrigin-RevId: 252585028

google/riegeli,google/riegeli,google/riegeli,google/riegeli