Also mark the case of an uninitialised memory view slice as "unlikely()" when...

Also mark the case of an uninitialised memory view slice as "unlikely()" when refcounting it since it's most probably not a performance critical case.

Cython/Utility/MemoryView_C.c

@@ -485,7 +485,7 @@ __Pyx_INC_MEMVIEW({{memviewslice_name}} *memslice, int have_gil, int lineno)
 {
     int first_time;
     struct {{memview_struct_name}} *memview = memslice->memview;
-    if (!memview || (PyObject *) memview == Py_None)
+    if (unlikely(!memview || (PyObject *) memview == Py_None))
         return; /* allow uninitialized memoryview assignment */
 
     if (unlikely(__pyx_get_slice_count(memview) < 0))
@@ -510,9 +510,8 @@ static CYTHON_INLINE void __Pyx_XDEC_MEMVIEW({{memviewslice_name}} *memslice,
     int last_time;
     struct {{memview_struct_name}} *memview = memslice->memview;
 
-    if (!memview ) {
-        return;
-    } else if ((PyObject *) memview == Py_None) {
+    if (unlikely(!memview || (PyObject *) memview == Py_None)) {
+        // we do not ref-count None
         memslice->memview = NULL;
         return;
     }