fix empty content

lib/api/readFile.js

@@ -41,8 +41,8 @@ module.exports = function readFile(filename, options, cb) {
       var callback = function(offset) {
         return function(err, content) {
           if (stop) return;
-          if (err) {
-            cb && cb(err);
+          if (err || !content) {
+            cb && cb(err || new Error('Empty content'));
             stop = true;
           } else {
             content.copy(result, offset.toNumber());