playback: fix playNext crash on last song of queue

Fixes OxygenCobalt/Auxio#735. ExoPlayer method for fetching next media
item returns C.INDEX_UNSET (-1) when used on the last song of a queue,
which is not a valid index for ExoPlayer.addMediaItems(). New code just
adds songs to the end of the queue if there isn't a next song.

app/src/main/java/org/oxycblt/auxio/playback/service/PlaybackServiceFragment.kt

@@ -362,7 +362,14 @@ constructor(
     }
 
     override fun playNext(songs: List<Song>, ack: StateAck.PlayNext) {
-        player.addMediaItems(player.nextMediaItemIndex, songs.map { it.toMediaItem() })
+        val nextIndex = player.nextMediaItemIndex
+
+        if (nextIndex == C.INDEX_UNSET) {
+            player.addMediaItems(songs.map { it.toMediaItem() })
+        } else {
+            player.addMediaItems(nextIndex, songs.map { it.toMediaItem() })
+        }
+
         playbackManager.ack(this, ack)
         deferSave()
     }