added isset($SID) to tep_href_link

added isset($SID) to tep_href_link so not to provide: Notice: Undefined variable: SID in...

I know notices are not shown is osCommerce because of :

error_reporting(E_ALL & ~E_NOTICE);

but that does not mean that we don't have to check for those.
Merge pull request #181 from gadlol/patch-6

added isset($SID) to tep_href_link