db/LightSong: make "real_uri" a pointer

src/DetachedSong.cxx

@@ -25,7 +25,7 @@
 
 DetachedSong::DetachedSong(const LightSong &other)
 	:uri(other.GetURI().c_str()),
-	 real_uri(other.real_uri),
+	 real_uri(other.real_uri != nullptr ? other.real_uri : ""),
 	 tag(*other.tag),
 	 mtime(other.mtime),
 	 start_ms(other.start_ms), end_ms(other.end_ms) {}

src/db/LightSong.hxx

@@ -48,13 +48,13 @@ struct LightSong {
 
 	/**
 	 * The "real" URI, the one to be used for opening the
-	 * resource.  If this attribute is empty, then #uri (and
+	 * resource.  If this attribute is nullptr, then #uri (and
 	 * #directory) shall be used.
 	 *
 	 * This attribute is used for songs from the database which
 	 * have a relative URI.
 	 */
-	std::string real_uri;
+	const char *real_uri;
 
 	/**
 	 * Must not be nullptr.

src/db/Song.cxx

@@ -102,6 +102,7 @@ Song::Export() const
 	dest.directory = parent->IsRoot()
 		? nullptr : parent->GetPath();
 	dest.uri = uri;
+	dest.real_uri = nullptr;
 	dest.tag = &tag;
 	dest.mtime = mtime;
 	dest.start_ms = start_ms;