macosx: prevent VLC from crashing after clearing the playlist and then opening a new file

1. After clearing the playlist and then opening a new file, the VLCPlayerController's current media item changes, which posts a VLCPlayerCurrentMediaItemChanged notification.
2. VLCPlaybackContinuityController receives this notification and calls inputItemChanged:.
3. This chain of events leads to a VLCInputItem object being deallocated.
4. The dealloc method in VLCInputItem.m calls input_item_Release(p_item).
5. The crash occurs because p_item is NULL, and input_item_Release doesn't handle NULL values, leading to a crash when it tries to access memory at an invalid address.