fix crash on end of playlist

apparently playlist_pos() can and will change from >0 to -1 between
those two queries

src/ui/window.rs

@@ -518,18 +518,16 @@ mod imp {
         }
 
         fn song(&self) -> Option<Song> {
-            if self.obj().playlist_pos() < 0 {
-                None
-            } else {
-                let song: Song = self
-                    .obj()
+            match self.obj().playlist_pos() {
+                playlist_pos if playlist_pos < 0 => None,
+                playlist_pos => Some(
+                    self.obj()
                         .playlist_model()
-                    .item(self.obj().playlist_pos() as u32)
+                        .item(playlist_pos as u32)
                         .unwrap()
                         .dynamic_cast()
-                    .unwrap();
-
-                Some(song)
+                        .unwrap(),
+                ),
             }
         }