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")
