wallet: Add asserts to detect unset transaction height values

Also document GetTxDepthInMainChain preconditions better

src/wallet/wallet.cpp

@@ -3303,8 +3303,10 @@ int CWallet::GetTxDepthInMainChain(const CWalletTx& wtx) const
 {
     AssertLockHeld(cs_wallet);
     if (auto* conf = wtx.state<TxStateConfirmed>()) {
+        assert(conf->confirmed_block_height >= 0);
         return GetLastBlockHeight() - conf->confirmed_block_height + 1;
     } else if (auto* conf = wtx.state<TxStateConflicted>()) {
+        assert(conf->conflicting_block_height >= 0);
         return -1 * (GetLastBlockHeight() - conf->conflicting_block_height + 1);
     } else {
         return 0;

src/wallet/wallet.h

@@ -503,6 +503,13 @@ class CWallet final : public WalletStorage, public interfaces::Chain::Notificati
      * <0  : conflicts with a transaction this deep in the blockchain
      *  0  : in memory pool, waiting to be included in a block
      * >=1 : this many blocks deep in the main chain
+     *
+     * Preconditions: it is only valid to call this function when the wallet is
+     * online and the block index is loaded. So this cannot be called by
+     * bitcoin-wallet tool code or by wallet migration code. If this is called
+     * without the wallet being online, it won't be able able to determine the
+     * the height of the last block processed, or the heights of blocks
+     * referenced in transaction, and might cause assert failures.
      */
     int GetTxDepthInMainChain(const CWalletTx& wtx) const EXCLUSIVE_LOCKS_REQUIRED(cs_wallet);
     bool IsTxInMainChain(const CWalletTx& wtx) const EXCLUSIVE_LOCKS_REQUIRED(cs_wallet)