fix: memory corruption due to accessing out of bounds output vector index

 In numpy_scan.cpp, the OBJECT/STRING scan path uses pos = i +
 offset (the absolute row index in the source array) as the output
 vector index for setNull() when handling None or NaN values:

 ```cpp
   // BEFORE (buggy):
   outputVector->setNull(pos, true /* isNull */);  // pos = i + offset
 ```

 The output vector is sized for DEFAULT_VECTOR_CAPACITY (= 2048)
 entries and expects 0-based indices within each batch. But pos is
 the absolute dataframe row index. When the COPY spans multiple
 batches (i.e., when offset >= DEFAULT_VECTOR_CAPACITY), pos exceeds
 the null mask buffer, causing a heap buffer overrun → the segfault
 / free(): invalid next size / munmap_chunk(): invalid pointer
 crashes seen on Linux.

Author: adsharma

src_cpp/numpy/numpy_scan.cpp

@@ -172,7 +172,7 @@ void NumpyScan::scan(PandasColumnBindData* bindData, uint64_t count, uint64_t of
                 !py::isinstance<py::str>(val)) {
                 if (val == Py_None ||
                     (py::isinstance<py::float_>(val) && std::isnan(PyFloat_AsDouble(val)))) {
-                    outputVector->setNull(pos, true /* isNull */);
+                    outputVector->setNull(i, true /* isNull */);
                     continue;
                 }
                 if (!py::isinstance<py::str>(val)) {