HTLC timeout transaction sweep is never run due to integer underflow about lightning

[rustyrussell]

This cosmetic only, and has been there for a long time. I keep meaning to fix it, but nobody complained before so I figured I'm the only one looking at the logs.

[daywalker90]

I've seen this before on a vanilla 23.11 #6951 (comment)

[ksedgwic]

I've got one too on:

 ab4ea82638b1d49e206e6637ae2211010534bf32 lightning (v24.02.1-18-gab4ea8263)
 206a1f23c343b5f3e70f0091fad57f5bae998797 vls (v0.11.0-73-g206a1f23)

         "state_changes": [
           {
              "timestamp": "2024-04-04T06:42:00.804Z",
              "old_state": "CHANNELD_AWAITING_LOCKIN",
              "new_state": "CHANNELD_NORMAL",
              "cause": "user",
              "message": "Lockin complete"
           },
           {
              "timestamp": "2024-04-25T03:48:08.845Z",
              "old_state": "CHANNELD_NORMAL",
              "new_state": "AWAITING_UNILATERAL",
              "cause": "protocol",
              "message": "Offered HTLC 506 SENT_ADD_ACK_REVOCATION cltv 2673110 hit deadline"
           },
           {
              "timestamp": "2024-04-25T03:48:43.696Z",
              "old_state": "AWAITING_UNILATERAL",
              "new_state": "FUNDING_SPEND_SEEN",
              "cause": "onchain",
              "message": "Onchain funding spend"
           },
           {
              "timestamp": "2024-04-25T03:48:47.086Z",
              "old_state": "FUNDING_SPEND_SEEN",
              "new_state": "ONCHAIN",
              "cause": "onchain",
              "message": "Onchain init reply"
           },
           
... lots of last repeated ...

           {
              "timestamp": "2024-05-12T23:11:33.667Z",
              "old_state": "FUNDING_SPEND_SEEN",
              "new_state": "ONCHAIN",
              "cause": "onchain",
              "message": "Onchain init reply"
           }
        ],
        "status": [
           "ONCHAIN:Tracking our own unilateral close",
           "ONCHAIN:3 outputs unresolved: in 4294825093 blocks will spend OUR_HTLC (26ede09939e5528593b6f4d57ebf93a3ab86b763094f1a5fcf3df0862e62a080:2) using OUR_HTLC_TIMEOUT_TX"
        ],

Logs believed to be available, anything you want me to look for?