Fixed: Parsing RSS with null values

(cherry picked from commit 8175f19442273e13517b2d507e7a4c33587f1b15)
(cherry picked from commit 6842c561ea36d1d2661cb87c517d6c155d65c4d3)

Closes #982

src/NzbDrone.Core/Indexers/RssParser.cs

@@ -272,9 +272,9 @@ namespace NzbDrone.Core.Indexers
                                      {
                                          return new RssEnclosure
                                          {
-                                             Url = v.Attribute("url").Value,
-                                             Type = v.Attribute("type").Value,
-                                             Length = (long)v.Attribute("length")
+                                             Url = v.Attribute("url")?.Value,
+                                             Type = v.Attribute("type")?.Value,
+                                             Length = v.Attribute("length")?.Value?.ParseInt64() ?? 0
                                          };
                                      }
                                      catch (Exception e)