fs: return null error on readFile() success

This commit ensures that readFile() callsback with a null
error consistently on success.

PR-URL: #3740
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Evan Lucas <evanlucas@me.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>

lib/fs.js

@@ -405,7 +405,7 @@ function readFileAfterClose(err) {
 }
 
 function tryToString(buf, encoding, callback) {
-  var e;
+  var e = null;
   try {
     buf = buf.toString(encoding);
   } catch (err) {