db/simple/Song: make "parent" a reference, not a pointer

src/db/plugins/simple/Directory.cxx

@@ -167,7 +167,7 @@ Directory::AddSong(SongPtr song) noexcept
 {
 	assert(holding_db_lock());
 	assert(song != nullptr);
-	assert(song->parent == this);
+	assert(&song->parent == this);
 
 	songs.push_back(*song.release());
 }
@@ -177,7 +177,7 @@ Directory::RemoveSong(Song *song) noexcept
 {
 	assert(holding_db_lock());
 	assert(song != nullptr);
-	assert(song->parent == this);
+	assert(&song->parent == this);
 
 	songs.erase(songs.iterator_to(*song));
 }
@@ -189,7 +189,7 @@ Directory::FindSong(const char *name_utf8) const noexcept
 	assert(name_utf8 != nullptr);
 
 	for (auto &song : songs) {
-		assert(song.parent == this);
+		assert(&song.parent == this);
 
 		if (song.uri == name_utf8)
 			return &song;

src/db/plugins/simple/Song.cxx

@@ -26,7 +26,7 @@
 
 inline
 Song::Song(StringView _uri, Directory &_parent) noexcept
-	:parent(&_parent), uri(_uri.data, _uri.size)
+	:parent(_parent), uri(_uri.data, _uri.size)
 {
 }
 
@@ -56,10 +56,10 @@ Song::NewFile(const char *path, Directory &parent) noexcept
 std::string
 Song::GetURI() const noexcept
 {
-	if (parent->IsRoot())
+	if (parent.IsRoot())
 		return uri;
 	else {
-		const char *path = parent->GetPath();
+		const char *path = parent.GetPath();
 
 		std::string result;
 		result.reserve(strlen(path) + 1 + uri.length());
@@ -74,8 +74,8 @@ LightSong
 Song::Export() const noexcept
 {
 	LightSong dest(uri.c_str(), tag);
-	if (!parent->IsRoot())
-		dest.directory = parent->GetPath();
+	if (!parent.IsRoot())
+		dest.directory = parent.GetPath();
 	dest.mtime = mtime;
 	dest.start_time = start_time;
 	dest.end_time = end_time;

src/db/plugins/simple/Song.hxx

@@ -60,10 +60,9 @@ struct Song {
 	Tag tag;
 
 	/**
-	 * The #Directory that contains this song.  Must be
-	 * non-nullptr.
+	 * The #Directory that contains this song.
 	 */
-	Directory *const parent;
+	Directory &parent;
 
 	/**
 	 * The time stamp of the last file modification.  A negative