Merge #20344: wallet: fix scanning progress calculation for single block range

5e146022daa4336de94447e5b8e5418296286927 wallet: fix scanning progress calculation for single block range (Sebastian Falbesoner)

Pull request description:

  If the blockchain is rescanned for a single block (i.e. start and stop hashes are equal, and with that also the estimated start/stop verification progress values) the progress calculation could lead to a NaN value caused by a division by zero (0.0/0.0), resulting in an invalid JSON result for the `getwalletinfo` RPC.  This PR fixes this behaviour by setting the progress to zero in that special case. Fixes #20297.

  The behaviour can easily be reproduced by continuously running single block rescans in an endless loop, e.g. via
  ```bash
  #!/bin/bash
  while true
  do
      bitcoin-cli rescanblockchain $(bitcoin-cli getblockcount)
  done
  ```

  and at the same time perform some `getwalletinfo` RPCs.

  On the master branch, this leads to frequent invalid responses (tested on mainchain):
  ```
  $ bitcoin-cli getwalletinfo
  error: couldn't parse reply from server
  $ curl --user `cat ~/.bitcoin/.cookie` --data-binary '{"jsonrpc": "1.0", "id": "curltest", "method": "getwalletinfo", "params": []}' -H 'content-type: text/plain;' http://127.0.0.1:8332/
  {"result":{"walletname":"","walletversion":169900,"format":"bdb","balance":0.00000000,"unconfirmed_balance":0.00000000,"immature_balance":0.00000000,"txcount":0,"keypoololdest":1603677276,"keypoolsize":1000,"hdseedid":"3196e33ecb47c7130e6ca60f2f895f9259860dca","keypoolsize_hd_internal":1000,"paytxfee":0.00000000,"private_keys_enabled":true,"avoid_reuse":false,"scanning":{"duration":0,"progress":},"descriptors":false},"error":null,"id":"curltest"}
  ```
  (note that missing value for "progress" in the JSON result).

  On the PR branch, the behaviour doesn't occur anymore.

ACKs for top commit:
  MarcoFalke:
    review ACK 5e146022daa4336de94447e5b8e5418296286927
  promag:
    Core review ACK 5e146022daa4336de94447e5b8e5418296286927.

Tree-SHA512: f0e6aad5a6cd08b36c5fe820fff0ef26663229b39169a4dbe757f3c795a41cf5c69c9dc90efe7515675ae1059307f8971123781a0514d10704123a6f28b125ab

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