Merge bitcoin/bitcoin#22379: wallet: erase spkmans rather than setting to nullptr

b945a31 wallet: erase spkmans rather than setting to nullptr (Andrew Chow)

Pull request description:

  In many places in ScriptPubKeyMan managing code, we assume that the ScriptPubKeyMan being retrieved actually exists and is not a nullptr. Thus removing a ScriptPubKeyMan requires erasing the object from the map rather than setting it to a nullptr.

  This fixes a segmentation fault that can be reached with `test/functional/wallet_descriptors.py --descriptors`

ACKs for top commit:
  S3RK:
    ACK b945a31

Tree-SHA512: 344a4cf9b1c168428750c751dcd24c52032506f20c81977fe93c4b5307ea209de72bb62a9c5284820f225b03acdc9573fceb734833d29b82f49d5a799ddcaea7

Author: fanquake

src/wallet/wallet.cpp

@@ -3184,7 +3184,7 @@ void CWallet::LoadActiveScriptPubKeyMan(uint256 id, OutputType type, bool intern
     spk_mans[type] = spk_man;
 
     if (spk_mans_other[type] == spk_man) {
-        spk_mans_other[type] = nullptr;
+        spk_mans_other.erase(type);
     }
 
     NotifyCanGetAddressesChanged();
@@ -3201,7 +3201,7 @@ void CWallet::DeactivateScriptPubKeyMan(uint256 id, OutputType type, bool intern
         }
 
         auto& spk_mans = internal ? m_internal_spk_managers : m_external_spk_managers;
-        spk_mans[type] = nullptr;
+        spk_mans.erase(type);
     }
 
     NotifyCanGetAddressesChanged();