prop224: Fix coverity warnings from #20657 merge.

- Fix various ssize_t/size_t confusions in the tests.

- Fix a weird memset argument:
  "bad_memset: Argument -16 in memset loses precision in
  memset(&desc_two->blinded_kp.pubkey.pubkey, -16, 32UL)."

- Fix check_after_deref instance in check_state_line_for_service_rev_counter():
  "check_after_deref: Null-checking items suggests that it may be null,
  but it has already been dereferenced on all paths leading to the
  check."