Guard against a null file showing an exception in release rejections

Fixes #1755

src/NzbDrone.Core/DecisionEngine/Specifications/CutoffSpecification.cs

@@ -1,4 +1,4 @@
-using System.Linq;
+using System.Linq;
 using NLog;
 using NzbDrone.Core.IndexerSearch.Definitions;
 using NzbDrone.Core.Parser.Model;
@@ -22,8 +22,13 @@ namespace NzbDrone.Core.DecisionEngine.Specifications
         {
             foreach (var file in subject.Episodes.Where(c => c.EpisodeFileId != 0).Select(c => c.EpisodeFile.Value))
             {
-                _logger.Debug("Comparing file quality with report. Existing file is {0}", file.Quality);
+                if (file == null)
+                {
+                    _logger.Debug("File is no longer avaialble, skipping this file.");
+                    continue;
+                }
 
+                _logger.Debug("Comparing file quality with report. Existing file is {0}", file.Quality);
                 
                 if (!_qualityUpgradableSpecification.CutoffNotMet(subject.Series.Profile, file.Quality, subject.ParsedEpisodeInfo.Quality))
                 {

src/NzbDrone.Core/DecisionEngine/Specifications/UpgradeDiskSpecification.cs

@@ -1,4 +1,4 @@
-using System.Linq;
+using System.Linq;
 using NLog;
 using NzbDrone.Core.IndexerSearch.Definitions;
 using NzbDrone.Core.Parser.Model;
@@ -22,6 +22,12 @@ namespace NzbDrone.Core.DecisionEngine.Specifications
         {
             foreach (var file in subject.Episodes.Where(c => c.EpisodeFileId != 0).Select(c => c.EpisodeFile.Value))
             {
+                if(file == null)
+                {
+                    _logger.Debug("File is no longer avaialble, skipping this file.");
+                    continue;
+                }
+
                 _logger.Debug("Comparing file quality with report. Existing file is {0}", file.Quality);
 
                 if (!_qualityUpgradableSpecification.IsUpgradable(subject.Series.Profile, file.Quality, subject.ParsedEpisodeInfo.Quality))