Delete None slices properly

Cython/Utility/MemoryView_C.c

@@ -450,8 +450,12 @@ static CYTHON_INLINE void __Pyx_XDEC_MEMVIEW({{memviewslice_name}} *memslice,
     int last_time;
     struct {{memview_struct_name}} *memview = memslice->memview;
 
-    if (!memview || (PyObject *) memview == Py_None)
+    if (!memview ) {
+        return;
+    } else if ((PyObject *) memview == Py_None) {
+        memslice->memview = NULL;
         return;
+    }
 
     if (memview->acquisition_count <= 0)
         __pyx_fatalerror("Acquisition count is %d (line %d)",

tests/run/memslice.pyx

@@ -2185,3 +2185,20 @@ def test_noneslice_ext_attr():
 
         obj.m = None
         print obj.m
+
+@testcase
+def test_noneslice_del():
+    """
+    >>> test_noneslice_del()
+    Traceback (most recent call last):
+       ...
+    UnboundLocalError: local variable 'm' referenced before assignment
+    """
+    cdef int[10] a
+    cdef int[:] m = a
+
+    with cython.nonecheck(True):
+        m = None
+        del m
+        print m
+