Add test for offset computation in empty array

The first of the added tests failed because of bug #998, and the fix is
verified by the test. The second and third testcases did not have an
error, but were added for completeness.

The first testcase is by SparrowLii, from the initial mention of the
bug.

Author: bluss

tests/array-construct.rs

@@ -7,6 +7,7 @@
 
 use defmac::defmac;
 use ndarray::prelude::*;
+use ndarray::arr3;
 use ndarray::Zip;
 
 #[test]
@@ -164,6 +165,43 @@ fn test_ones() {
     assert_eq!(a, b);
 }
 
+#[test]
+fn test_from_shape_empty_with_neg_stride() {
+    // Issue #998, negative strides for an axis where it doesn't matter.
+    let s = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13];
+    let v = s[..12].to_vec();
+    let v_ptr = v.as_ptr();
+    let a = Array::from_shape_vec((2, 0, 2).strides((1, -4isize as usize, 2)), v).unwrap();
+    assert_eq!(a, arr3(&[[[0; 2]; 0]; 2]));
+    assert_eq!(a.as_ptr(), v_ptr);
+}
+
+#[test]
+fn test_from_shape_with_neg_stride() {
+    // Issue #998, negative strides for an axis where it doesn't matter.
+    let s = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13];
+    let v = s[..12].to_vec();
+    let v_ptr = v.as_ptr();
+    let a = Array::from_shape_vec((2, 1, 2).strides((1, -4isize as usize, 2)), v).unwrap();
+    assert_eq!(a, arr3(&[[[0, 2]],
+                         [[1, 3]]]));
+    assert_eq!(a.as_ptr(), v_ptr);
+}
+
+#[test]
+fn test_from_shape_2_2_2_with_neg_stride() {
+    // Issue #998, negative strides for an axis where it doesn't matter.
+    let s = [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13];
+    let v = s[..12].to_vec();
+    let v_ptr = v.as_ptr();
+    let a = Array::from_shape_vec((2, 2, 2).strides((1, -4isize as usize, 2)), v).unwrap();
+    assert_eq!(a, arr3(&[[[4, 6],
+                          [0, 2]],
+                         [[5, 7],
+                          [1, 3]]]));
+    assert_eq!(a.as_ptr(), v_ptr.wrapping_add(4));
+}
+
 #[should_panic]
 #[test]
 fn deny_wraparound_zeros() {