fs: mkdtemp shouldn't crash if no callback passed

As it is, `fs.mkdtemp` crashes with a C++ assertion if the callback
function is not passed. This patch uses `maybeCallback` to create one,
if no callback function is passed.

PR-URL: https://github.com/nodejs/node/pull/6828
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

lib/fs.js

@@ -2000,7 +2000,8 @@ SyncWriteStream.prototype.destroy = function() {
 
 SyncWriteStream.prototype.destroySoon = SyncWriteStream.prototype.destroy;
 
-fs.mkdtemp = function(prefix, options, callback) {
+fs.mkdtemp = function(prefix, options, callback_) {
+  var callback = maybeCallback(callback_);
   if (!prefix || typeof prefix !== 'string')
     throw new TypeError('filename prefix is required');
 

test/parallel/test-fs-mkdtemp.js

@@ -25,3 +25,5 @@ fs.mkdtemp(
       assert(common.fileExists(folder));
     })
 );
+
+assert.doesNotThrow(() => fs.mkdtemp(path.join(common.tmpDir, 'bar-')));