Always consider 0-sized arrays as C- and F-contiguous (GH-3728)

Fixes https://github.com/cython/cython/issues/2093

Cython/Utility/MemoryView_C.c

@@ -347,18 +347,22 @@ static int __Pyx_ValidateAndInit_memviewslice(
     }
 
     /* Check axes */
-    for (i = 0; i < ndim; i++) {
-        spec = axes_specs[i];
-        if (unlikely(!__pyx_check_strides(buf, i, ndim, spec)))
-            goto fail;
-        if (unlikely(!__pyx_check_suboffsets(buf, i, ndim, spec)))
+    if (buf->len > 0) {
+        // 0-sized arrays do not undergo these checks since their strides are
+        // irrelevant and they are always both C- and F-contiguous.
+        for (i = 0; i < ndim; i++) {
+            spec = axes_specs[i];
+            if (unlikely(!__pyx_check_strides(buf, i, ndim, spec)))
+                goto fail;
+            if (unlikely(!__pyx_check_suboffsets(buf, i, ndim, spec)))
+                goto fail;
+        }
+
+        /* Check contiguity */
+        if (unlikely(buf->strides && !__pyx_verify_contig(buf, ndim, c_or_f_flag)))
             goto fail;
     }
 
-    /* Check contiguity */
-    if (unlikely(buf->strides && !__pyx_verify_contig(buf, ndim, c_or_f_flag)))
-        goto fail;
-
     /* Initialize */
     if (unlikely(__Pyx_init_memviewslice(memview, ndim, memviewslice,
                                          new_memview != NULL) == -1)) {

tests/memoryview/relaxed_strides.pyx

@@ -62,3 +62,25 @@ def test_zero_sized(array):
     """
     cdef double[::1] a = array
     return a
+
+def test_zero_sized_multidim_ccontig(array):
+    """
+    >>> contig = np.ascontiguousarray(np.zeros((4,4,4))[::2, 2:2, ::2])
+    >>> _ = test_zero_sized_multidim_ccontig(contig)
+
+    >>> a = np.zeros((4,4,4))[::2, 2:2, ::2]
+    >>> if NUMPY_HAS_RELAXED_STRIDES: _ = test_zero_sized_multidim_ccontig(a)
+    """
+    cdef double[:, :, ::1] a = array
+    return a
+
+def test_zero_sized_multidim_fcontig(array):
+    """
+    >>> contig = np.ascontiguousarray(np.zeros((4,4,4))[::2, 2:2, ::2])
+    >>> _ = test_zero_sized_multidim_fcontig(contig)
+
+    >>> a = np.zeros((4,4,4))[::2, 2:2, ::2]
+    >>> if NUMPY_HAS_RELAXED_STRIDES: _ = test_zero_sized_multidim_fcontig(a)
+    """
+    cdef double[::1, :, :] a = array
+    return a