Neaten-up the inner-loop logic.

Modules/_collectionsmodule.c

@@ -1053,13 +1053,13 @@ deque_index(dequeobject *deque, PyObject *args)
         }
     }
 
-    n = stop - i;
-    while (n--) {
+    n = stop - i + 1;
+    while (--n) {
         CHECK_NOT_END(b);
         item = b->data[index];
         cmp = PyObject_RichCompareBool(item, v, Py_EQ);
         if (cmp > 0)
-            return PyLong_FromSsize_t(stop - (n + 1));
+            return PyLong_FromSsize_t(stop - n);
         if (cmp < 0)
             return NULL;
         if (start_state != deque->state) {