Fix incorrect beahavior when dequeueing HEAD

Fix crash when trying to check if a non-existent download now exists
when queue is non-zero length

Author: ejv2

sound/queue.go

@@ -193,5 +193,5 @@ func DownloadAtHead(expect *data.QueueItem) bool {
 	mut.RLock()
 	defer mut.RUnlock()
 
-	return len(queue) != 0 && queue[head-1] == expect
+	return len(queue) != 0 && head-1 >= 0 && queue[head-1] == expect
 }