Fix: fs.readFile would execute callbacks twice

fs.readFile was executing a callback in a try..catch context, which is
a problem in itself. To make matters worse, it would re-execute the
same callback if there was an execution.

This patch fixes both of these problems.

lib/fs.js

@@ -73,10 +73,12 @@ fs.readFile = function (path, encoding_, callback) {
           binding.close(fd);
           if (encoding) {
             try {
-              callback(null, buffer.toString(encoding));
+              var str = buffer.toString(encoding);
             } catch (err) {
               callback(err);
+              return;
             }
+            callback(null, str);
           } else {
             callback(null, buffer);
           }