[core] Verifies valid and absolute HttpSeed urls on load

Invalid urls would fail to parse in Mode.DownloadLogic throwing an unhandled exception. This adds a check for a well formed and absolute uri before adding them to the HttpSeeds

src/MonoTorrent.Tests/Common/TorrentTest.cs

@@ -67,7 +67,11 @@ public void StartUp ()
                 { "created by", new BEncodedString ($"MonoTorrent/{VersionInfo.ClientVersion}") },
                 { "encoding", new BEncodedString ("UTF-8") },
                 { "info", CreateInfoDict () },
-                { "private", new BEncodedString ("1") }
+                { "private", new BEncodedString ("1") },
+                { "url-list", new BEncodedList() {
+                    new BEncodedString ("https://example.com/8/items/"),
+                    new BEncodedString ("/8/items/"), // this should be ignored on loading
+                } }
             };
             torrent = Torrent.Load (torrentInfo);
         }
@@ -256,6 +260,16 @@ public void Files ()
             Assert.AreEqual (80000, torrent.Files[3].Length);
         }
 
+        /// <summary>
+        ///
+        /// </summary>
+        [Test]
+        public void HttpSeeds ()
+        {
+            Assert.IsTrue (torrent.HttpSeeds.Count == 1);
+            Assert.AreEqual("https://example.com/8/items/", torrent.HttpSeeds[0]);
+        }
+
         [Test]
         public void InvalidPath ()
         {

src/MonoTorrent/MonoTorrent/Torrent.cs

@@ -13,10 +13,10 @@
 // distribute, sublicense, and/or sell copies of the Software, and to
 // permit persons to whom the Software is furnished to do so, subject to
 // the following conditions:
-// 
+//
 // The above copyright notice and this permission notice shall be
 // included in all copies or substantial portions of the Software.
-// 
+//
 // THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 // EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
 // MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
@@ -676,10 +676,14 @@ void LoadInternal (BEncodedDictionary torrentInformation, InfoHash infoHash)
 
                     case ("url-list"):
                         if (keypair.Value is BEncodedString httpSeedString) {
-                            HttpSeeds.Add (httpSeedString.Text);
+                            if (Uri.IsWellFormedUriString (httpSeedString.Text, UriKind.Absolute)) {
+                                HttpSeeds.Add (httpSeedString.Text);
+                            }
                         } else if (keypair.Value is BEncodedList httpSeedList) {
                             foreach (BEncodedString str in httpSeedList)
-                                HttpSeeds.Add (str.Text);
+                                if (Uri.IsWellFormedUriString (str.Text, UriKind.Absolute)) {
+                                    HttpSeeds.Add (str.Text);
+                                }
                         }
                         break;