walletdb: Consistently clear key and value streams before writing

Before writing data to the output key and value streams, make sure they
are cleared.

Author: achow101

src/wallet/bdb.cpp

@@ -777,6 +777,7 @@ bool BerkeleyBatch::ReadKey(DataStream&& key, DataStream& value)
     SafeDbt datValue;
     int ret = pdb->get(activeTxn, datKey, datValue, 0);
     if (ret == 0 && datValue.get_data() != nullptr) {
+        value.clear();
         value.write({AsBytePtr(datValue.get_data()), datValue.get_size()});
         return true;
     }

src/wallet/sqlite.cpp

@@ -409,6 +409,7 @@ bool SQLiteBatch::ReadKey(DataStream&& key, DataStream& value)
     // Leftmost column in result is index 0
     const std::byte* data{AsBytePtr(sqlite3_column_blob(m_read_stmt, 0))};
     size_t data_size(sqlite3_column_bytes(m_read_stmt, 0));
+    value.clear();
     value.write({data, data_size});
 
     sqlite3_clear_bindings(m_read_stmt);
@@ -495,6 +496,9 @@ DatabaseCursor::Status SQLiteCursor::Next(DataStream& key, DataStream& value)
         return Status::FAIL;
     }
 
+    key.clear();
+    value.clear();
+
     // Leftmost column in result is index 0
     const std::byte* key_data{AsBytePtr(sqlite3_column_blob(m_cursor_stmt, 0))};
     size_t key_data_size(sqlite3_column_bytes(m_cursor_stmt, 0));

src/wallet/test/util.cpp

@@ -100,6 +100,8 @@ DatabaseCursor::Status MockableCursor::Next(DataStream& key, DataStream& value)
     if (m_cursor == m_cursor_end) {
         return Status::DONE;
     }
+    key.clear();
+    value.clear();
     const auto& [key_data, value_data] = *m_cursor;
     key.write(key_data);
     value.write(value_data);
@@ -117,6 +119,7 @@ bool MockableBatch::ReadKey(DataStream&& key, DataStream& value)
     if (it == m_records.end()) {
         return false;
     }
+    value.clear();
     value.write(it->second);
     return true;
 }