add error handling

MediaBrowser.MediaEncoding/Encoder/MediaEncoder.cs

@@ -322,8 +322,12 @@ namespace MediaBrowser.MediaEncoding.Encoder
                 files = Directory.GetFiles(path, "*", SearchOption.AllDirectories);
 
                 ffmpegPath = files.FirstOrDefault(i => string.Equals(Path.GetFileNameWithoutExtension(i), "ffmpeg", StringComparison.OrdinalIgnoreCase));
+
+                if (!string.IsNullOrWhiteSpace(ffmpegPath))
+                {
                     ffprobePath = GetProbePathFromEncoderPath(ffmpegPath);
                 }
+            }
 
             return new Tuple<string, string>(ffmpegPath, ffprobePath);
         }