browser_element_focus: fix error with xul textboxes, xulrunner>=8
An error was dumped to console in XULRunner 8, if you open about:config in
a buffer and hit tab.
TypeError: elem.wrappedJSObject is undefined
chrome://conkeror/content/element.js:368
browser_element_focus([object Object],[object XULElement])@chrome://conkeror/content/element.js:368
...
Console error: [JavaScript Warning: "reference to undefined property elem.wrappedJSObject"
{file: "chrome://conkeror/content/element.js" line: 368}]
Category: chrome javascript