NULL passed to hook_form_alter() causing "Recoverable fatal error"

In comment_login_form_alter() is the module passing NULL as $form_state to hook_form_alter() in line 64. This is causing a fatal error with modules that require $form_state to be an array, for example xmlsitemap 6.x-2.0-beta2 (xmlsitemap_node.module line 161).

The attached patch is changing $null to be an array() which solves the conflict with xmlsitemap_node.module.