opcache: Call orig_post_shutdown_cb()

This was never called and resulted in this compiler warning:

```
./ext/opcache/ZendAccelerator.c:3720:15: warning: variable 'orig_post_shutdown_cb' set
      but not used [-Wunused-but-set-global]
 3720 | static void (*orig_post_shutdown_cb)(void);
      |               ^
```

Author: ndossche

ext/opcache/ZendAccelerator.c

@@ -3427,6 +3427,11 @@ static void (*orig_post_shutdown_cb)(void);
 
 static void accel_post_shutdown(void)
 {
+	/* Call original shutdown prior to detaching SHM, so it can access its memory safely. */
+	if (orig_post_shutdown_cb) {
+		orig_post_shutdown_cb();
+	}
+
 	zend_shared_alloc_shutdown();
 }