Fix null exception in other tmdb providers

MediaBrowser.Providers/Plugins/Tmdb/TV/TmdbSeasonProvider.cs

@@ -75,7 +75,7 @@ namespace MediaBrowser.Providers.Plugins.Tmdb.TV
 
                     result.Item.Overview = seasonInfo.Overview;
 
-                    if (seasonInfo.External_Ids.Tvdb_Id > 0)
+                    if (seasonInfo.External_Ids != null && seasonInfo.External_Ids.Tvdb_Id > 0)
                     {
                         result.Item.SetProviderId(MetadataProvider.Tvdb, seasonInfo.External_Ids.Tvdb_Id.Value.ToString(CultureInfo.InvariantCulture));
                     }

MediaBrowser.Providers/Plugins/Tmdb/TV/TmdbSeriesProvider.cs

@@ -92,7 +92,7 @@ namespace MediaBrowser.Providers.Plugins.Tmdb.TV
                 remoteResult.SetProviderId(MetadataProvider.Tmdb, obj.Id.ToString(_usCulture));
                 remoteResult.SetProviderId(MetadataProvider.Imdb, obj.External_Ids.Imdb_Id);
 
-                if (obj.External_Ids.Tvdb_Id > 0)
+                if (obj.External_Ids != null && obj.External_Ids.Tvdb_Id > 0)
                 {
                     remoteResult.SetProviderId(MetadataProvider.Tvdb, obj.External_Ids.Tvdb_Id.Value.ToString(_usCulture));
                 }