libsaria: Don't choose a random song if none are visible

This lead to a floating point exception and crashed Ocarina.  Oops...

Signed-off-by: Bryan Schumaker <bjschuma@gmail.com>

libsaria/playlist/next.cpp

@@ -47,7 +47,12 @@ namespace libsaria
 
 	void Playlist::random_next()
 	{
-		unsigned int n = rand() % apparent_size();
+		unsigned int n, num_visible = apparent_size();
+
+		if (num_visible == 0)
+			return;
+
+		n = rand() % num_visible;
 
 		for (unsigned int i = 0; i < n; i++) {
 			incr_iter(cur);