player/Control: initialize seek_time in EnqueueSongLocked()

Allows removing a few special cases in the player thread.

src/player/Control.hxx

@@ -417,6 +417,7 @@ private:
 		assert(next_song == nullptr);
 
 		next_song = song;
+		seek_time = SongTime::zero();
 		SynchronousCommand(PlayerCommand::QUEUE);
 	}