fs: properly handle fd passed to truncate()

Currently, fs.truncate() silently fails when a file descriptor
is passed as the first argument. This commit changes this
behavior to properly call fs.ftruncate(). This commit also
adds proper type checking to the callback provided to
makeCallback().

PR-URL: nodejs#9161
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Timothy J Fontaine <tjfontaine@gmail.com>

lib/fs.js

@@ -87,10 +87,14 @@ function maybeCallback(cb) {
 // for callbacks that are passed to the binding layer, callbacks that are
 // invoked from JS already run in the proper scope.
 function makeCallback(cb) {
-  if (!util.isFunction(cb)) {
+  if (util.isNullOrUndefined(cb)) {
     return rethrow();
   }
 
+  if (!util.isFunction(cb)) {
+    throw new TypeError('callback must be a function');
+  }
+
   return function() {
     return cb.apply(null, arguments);
   };