Embed won't work when content has just the ad tag

This is because the function that searches for tags start from position one. The issue should be solved changing file embed\ad_embed.module, line 244 as follow:

Replace:

$pos = strpos($text, $open, $pos+1);
if ($pos) {

With:

$pos = strpos($text, $open, $pos);
if ($pos !== FALSE) {

Thanks for this great Drupal module,
Luigi