Toolbar shows a warning on a second call

[SergeyBiryukov]

Slight regression from 3.2.

After upgrading to 3.3, some people saw the warning near the Toolbar:

Notice: Trying to get property of non-object in wp-includes/class-wp-admin-bar.php on line 342
Warning: Invalid argument supplied for foreach() in wp-includes/class-wp-admin-bar.php on line 342

• ​http://wordpress.org/support/topic/receiving-an-invalid-argument-error-on-homepage
• ​http://wordpress.org/support/topic/no-admin-toolbar-on-website-page-after-upgrade-to-33
• ​http://wordpress.org/support/topic/admin-bar-not-loading-blog-page-not-admin-side
• ​http://wordpress.org/support/topic/message-appears-across-top-of-site

This happens if a theme has two wp_footer() calls, or both do_action('wp_footer') and wp_footer() (which of course is wrong and weird, but didn't produce a warning in 3.2).

The problem is that $wp_admin_bar->render() doesn't check the result of _bind() (which returns nothing on a second call) before calling _render():
​http://core.trac.wordpress.org/browser/tags/3.3/wp-includes/class-wp-admin-bar.php#L197

[nacin]

In [19638]:

Make sure that _render() is only called with a valid root object. As in, render() can only be validly called once and must not fail on the second call. props SergeyBiryukov, see #19642 for trunk.

[nacin]

Marking as commit for the 3.3 branch, after additional review.

[ryan]

In [19644]:

Make sure that _render() is only called with a valid root object. As in, render() can only be validly called once and must not fail on the second call. props SergeyBiryukov, fixes #19642 for 3.3