Fix _.extend regression in IE < 9.

Former-commit-id: 65632fbbf3aaba0d9cd8578300f8025e311faff0

lodash.js

@@ -248,11 +248,12 @@
     'init': 'object',
     'top':
       'for (var source, sourceIndex = 1, length = arguments.length; sourceIndex < length; sourceIndex++) {\n' +
-      '  source = arguments[sourceIndex]',
+      '  source = arguments[sourceIndex];\n' +
+      (hasDontEnumBug ? '  if (source) {' : ''),
     'loopExp': 'index in source',
     'useHas': false,
     'inLoop': 'object[index] = source[index]',
-    'bottom': '}'
+    'bottom': (hasDontEnumBug ? '  }\n' : '') + '}'
   };
 
   /** Reusable iterator options for `filter`  and `reject` */

perf/index.html

@@ -30,7 +30,7 @@
 			}());
 
 			window.onload = function() {
-				var sibling = document.scripts[0],
+				var sibling = document.getElementsByTagName('script')[0],
 				    script = document.createElement('script');
 
 				document.getElementById('FirebugUI').style.height = '100%';

test/backbone.html

@@ -5,7 +5,10 @@
 		<title>Backbone Test Suite</title>
 		<link rel="stylesheet" href="../vendor/backbone/test/vendor/qunit.css">
 		<style>
-			body > #qunit-header, #jslitmus {
+			body > #qunit-header {
+				display: none;
+			}
+			#jslitmus {
 				display: none;
 			}
 		</style>

test/test.js

@@ -149,6 +149,14 @@
   QUnit.module('lodash.extend');
 
   (function() {
+    test('should not error on `null` or `undefined` sources (test in IE < 9)', function() {
+      try {
+        deepEqual(_.extend({}, null, undefined, { 'a': 1 }), { 'a': 1 });
+      } catch(e) {
+        ok(false);
+      }
+    });
+
     test('skips the prototype property of functions (test in Firefox < 3.6, Opera > 9.50 - Opera < 11.60, and Safari < 5.1)', function() {
       function Foo() {}
       Foo.prototype.c = 3;