Process non-empty environment variables in get_cgi_filename

[kapouer]

Easier to debug when nginx sends empty variables.
Allows unsetting SCRIPT_FILENAME like in :

include /etc/nginx/fastcgi_params;
fastcgi_param   SCRIPT_FILENAME         "";
fastcgi_param   SCRIPT_NAME             $fastcgi_script_name;
fastcgi_param   DOCUMENT_ROOT           $document_root;

[Lekensteyn]

You can also use *p instead of strlen(p) > 0 to check whether a string is empty or not.