@ign
Replace the line of code you posted with the following..
foreach( $menu as $menu_key => $menu_data ) :
if( 'edit.php' != $menu_data[2] )
continue;
$menu[$menu_key][0] .= " <span class="update-plugins count-$pending_count"><span class="plugin-count">" . number_format_i18n($pending_count) . '</span></span>';
endforeach;
..that should avoid the need to know the specific key.. (let me know if any problems)..
Hope that helps.. 🙂