Remove duplicate session_start() if HTTP_USER_AGENT is not defined.

Debug INFO: Undefined index: HTTP_USER_AGENT in \catalog\includes\application_top.php on line 131

Warning: session_start(): Cannot send session cookie - headers already sent by (output started at \includes\application_top.php:131) in \includes\functions\sessions.php on line 90
Merge pull request #240 from acidvertigo/patch-26

Remove duplicate session_start() if HTTP_USER_AGENT is not defined.