Change 3 callbacks to explicitly return null as an error if there is no error

lib/smb2.js

@@ -130,7 +130,7 @@ proto.readdir = function(path, cb){
         // SMB2 close directory
         else SMB2Request('close', file, connection, function(err){
           cb && cb(
-            err
+            null
           , files
               .map(function(v){ return v.Filename }) // get the filename only
               .filter(function(v){ return v!='.' && v!='..' }) // remove '.' and '..' values
@@ -206,7 +206,7 @@ proto.readFile = function(filename, options, cb){
               if(options.encoding){
                 result = result.toString(options.encoding);
               }
-              cb && cb(err, result);
+              cb && cb(null, result);
             })
           }
         }
@@ -415,7 +415,7 @@ proto.unlink = function(path, cb){
             if(err) cb && cb(err);
             // SMB2 close directory
             else SMB2Request('close', file, connection, function(err){
-              cb && cb(err, files);
+              cb && cb(null, files);
             });
           });
         });