[bookmarklet] fix color restoration on mouse out

if we got multiple mouseenter-events (was happening sometimes), it was
possible that the oldbg helper was set to the TA green background at
which point it was impossible for that off() function to restore the
original background

public/bookmarklet.js

@@ -47,7 +47,9 @@
 
       $('input[type=text]').live('mouseenter.ta', function(){
         var el = $(this);
-        el[0].oldbg = el.css('background-color');
+
+        if (!el[0].oldbg)
+          el[0].oldbg = el.css('background-color');
         el
           .css('background-color', '#72a100')
           .addClass('__ta_highlight');