db/simple: reorder checks in assert() to fix assertion failure

`light_song.Get()` could cause an assertion failure because the
`Manual<>` object must not be used if uninitialized.

Regression by commit ebc006ab52

src/db/plugins/simple/SimpleDatabasePlugin.cxx

@@ -251,7 +251,7 @@ void
 SimpleDatabase::ReturnSong(gcc_unused const LightSong *song) const noexcept
 {
 	assert(song != nullptr);
-	assert(song == &light_song.Get() || song == prefixed_light_song);
+	assert(song == prefixed_light_song || song == &light_song.Get());
 
 	if (prefixed_light_song != nullptr) {
 		delete prefixed_light_song;