Fix linker stack null reference (#590)

Author: tmcw

lib/output/util/linker_stack.js

@@ -40,6 +40,7 @@ function firstPass(fns, input) {
  * @returns {Function} linker method
  */
 function LinkerStack(config) {
+  config = config || {};
   this.stack = [];
 
   if (config.defaultGlobals !== false) {