Merge bitcoin/bitcoin#23333: wallet: fix segfault by avoiding invalid default-ctored `external_spk_managers` entry

6911ab95f19d2b1f60f2d0b2f3961fa6639d4f31 wallet: fix segfault by avoiding invalid default-ctored `external_spk_managers` entry (Sebastian Falbesoner)

Pull request description:

  Fixes #23321 (bug reported by Josef Vondrlik (josef-v)).

  In the method `CWallet::LoadActiveScriptPubKeyMan`, the map `external_spk_managers` (or `internal_spk_managers`, if parameter `internal` is false) is accessed via std::map::operator[], which means that a default-ctored entry is created with a null-pointer as value, if the key doesn't exist.  As soon as this value is dereferenced, a segmentation fault occurs, e.g. in `CWallet::KeypoolCountExternalKeys`.

  The bevaviour can be reproduced by the following steps (starting with empty regtest datadir):

  ```
  $ ./src/bitcoind -regtest -daemon
  $ ./src/bitcoin-cli -regtest -named createwallet_name=wallet descriptors=true blank=true
  $ cat regtest-descriptors.txt
  [
    {
      "desc": "tr([e4445899/49'/1'/0']tprv8ZgxMBicQKsPd8jCeBWsYLEoWxbVgzJDatJ7XkwQ6G3uF4FsHuaziHQ5JZAW4K515nj6kVVwPaNWZSMEcR7aFCwL4tQqTcaoprMKTTtm6Zg/1/*)#mr3llm7f",
      "timestamp": 1634652324,
      "active": true,
      "internal": true,
      "range": [
        0,
        999
      ],
      "next": 0
    }
  ]
  $ ./src/bitcoin-cli -regtest importdescriptors "$(cat regtest-descriptors.txt)"
  [
    {
      "success": true
    }
  ]
  $ ./src/bitcoin-cli -regtest getwalletinfo
  error: timeout on transient error: Could not connect to the server 127.0.0.1:18443 (error code 1 - "EOF reached")
  ```

ACKs for top commit:
  achow101:
    Code Review ACK 6911ab95f19d2b1f60f2d0b2f3961fa6639d4f31
  lsilva01:
    Tested ACK 6911ab9  on Ubuntu 20.04.
  instagibbs:
    ACK 6911ab95f19d2b1f60f2d0b2f3961fa6639d4f31

Tree-SHA512: 76aa96847cf2739413fb68fb902afef0b3ab9381178dd62fb0abac69f853f1f6523d73c60e610375b9a7730f275eda9162503b89f5be6e6e349a8d047b59c8dc

0 files changed, 0 insertions, 0 deletions