BUG: Fix unpickling an empty ndarray with a none-zero dimension (#21067)

Changing num to the number of bytes in the input array, PyArray_NBYTES(self). Solves #21009.

* Fixing nbyte size in methods.c:memcpy

* Adding a test

* Re-adding removed newline

* Shrinking the test array to save memory

numpy/core/src/multiarray/methods.c

@@ -2175,7 +2175,7 @@ array_setstate(PyArrayObject *self, PyObject *args)
                 Py_DECREF(typecode);
             }
             else {
-                memcpy(PyArray_DATA(self), datastr, num);
+                memcpy(PyArray_DATA(self), datastr, PyArray_NBYTES(self));
             }
             PyArray_ENABLEFLAGS(self, NPY_ARRAY_OWNDATA);
             fa->base = NULL;

numpy/core/tests/test_multiarray.py

@@ -1640,6 +1640,15 @@ def test_pickle(self):
 
                 assert_equal(zs.dtype, zs2.dtype)
 
+    def test_pickle_empty(self):
+        """Checking if an empty array pickled and un-pickled will not cause a
+        segmentation fault"""
+        arr = np.array([]).reshape(999999, 0)
+        pk_dmp = pickle.dumps(arr)
+        pk_load = pickle.loads(pk_dmp)
+
+        assert pk_load.size == 0
+
     @pytest.mark.skipif(pickle.HIGHEST_PROTOCOL < 5,
                         reason="requires pickle protocol 5")
     def test_pickle_with_buffercallback(self):