Assert that CWallet::SyncMetaData finds oldest transaction.

This fixes one of the Clang static analyzer warnings mentioned in #9573.

src/wallet/wallet.cpp

@@ -569,6 +569,9 @@ void CWallet::SyncMetaData(std::pair<TxSpends::iterator, TxSpends::iterator> ran
             copyFrom = &mapWallet[hash];
         }
     }
+
+    assert(copyFrom);
+
     // Now copy data from copyFrom to rest:
     for (TxSpends::iterator it = range.first; it != range.second; ++it)
     {