fixed pure AMD define webpack#32

Author: sokra

lib/parse.js

@@ -351,6 +351,7 @@ function walkExpression(options, context, expression) {
 			expression.arguments.length == 1 &&
 			expression.callee.type === "Identifier" &&
 			expression.callee.name === "define") {
+			context.requires = context.requires || [];
 			context.requires.push({
 				name: "__webpack_amd_define",
 				append: "(module)",