Fix downloading releases without an indexer

(cherry picked from commit ca8b26138e3ebd793cc1a5fd51522ce3eda8a2e1)

Closes #3598

src/NzbDrone.Core/Download/DownloadService.cs

@@ -63,7 +63,6 @@ namespace NzbDrone.Core.Download
 
             // Get the seed configuration for this release.
             remoteAlbum.SeedConfiguration = _seedConfigProvider.GetSeedConfiguration(remoteAlbum);
-            var indexer = _indexerFactory.GetInstance(_indexerFactory.Get(remoteAlbum.Release.IndexerId));
 
             // Limit grabs to 2 per second.
             if (remoteAlbum.Release.DownloadUrl.IsNotNullOrWhiteSpace() && !remoteAlbum.Release.DownloadUrl.StartsWith("magnet:"))
@@ -72,6 +71,13 @@ namespace NzbDrone.Core.Download
                 _rateLimitService.WaitAndPulse(url.Host, TimeSpan.FromSeconds(2));
             }
 
+            IIndexer indexer = null;
+
+            if (remoteAlbum.Release.IndexerId > 0)
+            {
+                indexer = _indexerFactory.GetInstance(_indexerFactory.Get(remoteAlbum.Release.IndexerId));
+            }
+
             string downloadClientId;
             try
             {

src/NzbDrone.Core/Download/TorrentClientBase.cs

@@ -127,7 +127,7 @@ namespace NzbDrone.Core.Download
 
             try
             {
-                var request = indexer.GetDownloadRequest(torrentUrl);
+                var request = indexer?.GetDownloadRequest(torrentUrl) ?? new HttpRequest(torrentUrl);
                 request.RateLimitKey = remoteAlbum?.Release?.IndexerId.ToString();
                 request.Headers.Accept = "application/x-bittorrent";
                 request.AllowAutoRedirect = false;

src/NzbDrone.Core/Download/UsenetClientBase.cs

@@ -44,7 +44,7 @@ namespace NzbDrone.Core.Download
 
             try
             {
-                var request = indexer.GetDownloadRequest(url);
+                var request = indexer?.GetDownloadRequest(url) ?? new HttpRequest(url);
                 request.RateLimitKey = remoteAlbum?.Release?.IndexerId.ToString();
 
                 // TODO: Look into moving download request handling to indexer