Update Emby.Server.Implementations/Library/LibraryManager.cs

Co-Authored-By: Bond-009 <bond.009@outlook.com>

Emby.Server.Implementations/Library/LibraryManager.cs

@@ -143,7 +143,7 @@ namespace Emby.Server.Implementations.Library
         public bool IsScanRunning { get; private set; }
 
         private IServerApplicationHost _appHost;
-        private IMediaEncoder _mediaEncoder;
+        private readonly IMediaEncoder _mediaEncoder;
 
         /// <summary>
         /// The _library items cache