Fix a reference leak for None when releasing a memoryview object.

Closes GH-3023.

Cython/Utility/MemoryView.pyx

@@ -372,6 +372,10 @@ cdef class memoryview(object):
     def __dealloc__(memoryview self):
         if self.obj is not None:
             __Pyx_ReleaseBuffer(&self.view)
+        elif (<__pyx_buffer *> &self.view).obj == Py_None:
+            # Undo the incref in __cinit__() above.
+            (<__pyx_buffer *> &self.view).obj = NULL
+            Py_DECREF(Py_None)
 
         cdef int i
         global __pyx_memoryview_thread_locks_used

tests/memoryview/memoryview.pyx

@@ -698,6 +698,17 @@ def assign_temporary_to_object(object[:] mslice):
     buf = mslice
     buf[1] = {3-2: 2+(2*4)-2}
 
+
+def test_pyview_of_memview(int[:] ints):
+    """
+    >>> A = IntMockBuffer(None, [1, 2, 3])
+    >>> view = test_pyview_of_memview(A)
+    >>> len(memoryview(view))
+    3
+    """
+    return ints
+
+
 def test_generic_slicing(arg, indirect=False):
     """
     Test simple slicing