Use offset with length for statement query

[smvv]

CI job release fails in Install sdist on main with:

In member function ‘std::string duckdb::DuckDBPyStatement::Query() const’: src/pystatement.cpp:32:35:
error: ‘class duckdb::SQLStatement’ has no member named ‘stmt_length’
     32 |         auto &length = statement->stmt_length;
        |                                   ^~~~~~~~~~~

[evertlammerts]

Thanks for the fix! There's another breaking change in core's main, so I fixed both in #575. Feel free to commandeer :-)