This is a small folllow-up to PR16528 ("Native Descriptor Wallets using DescriptorScriptPubKeyMan")
Before this change bool m_internal was left uninitialized when using the DescriptorScriptPubKeyMan(WalletStorage&, WalletDescriptor&) ctor.
The same goes for the now initialized integers which were left uninitialized when using the WalletDescriptor() ctor.
wallet: Make sure no DescriptorScriptPubKeyMan members are uninitialized after construction
wallet: Make sure no WalletDescriptor members are uninitialized after construction
This is a backport of Core PR18782