[core] Generate a proper file path as the fallback

The API expects this to be the path to a specific file,
so when we autogenerate a value ensure we actually
generate a full path.

src/MonoTorrent/MonoTorrent.Client/Managers/TorrentManager.cs

@@ -355,7 +355,7 @@ public TorrentManager (MagnetLink magnetLink, string savePath, TorrentSettings s
 
             InfoHash = magnetLink.InfoHash;
             Settings = settings;
-            this.torrentSave = string.IsNullOrEmpty (torrentSave) ? Environment.CurrentDirectory : Path.GetFullPath (torrentSave);
+            this.torrentSave = string.IsNullOrEmpty (torrentSave) ? Path.Combine (Environment.CurrentDirectory, "metadata", $"{InfoHash.ToHex ()}.torrent") : Path.GetFullPath (torrentSave);
             var announces = new List<IList<string>> ();
             if (magnetLink.AnnounceUrls != null)
                 announces.Add (magnetLink.AnnounceUrls);