Referer: fixed $invalid_referer.

Referer: fixed $invalid_referer.

The variable was considered non-existent in the absence of any
valid_referers directives.

Given the following config snippet,

    location / {
        return 200 $invalid_referer;
    }

    location /referer {
        valid_referers server_names;
    }

"location /" should work identically and independently on other
"location /referer".

The fix is to always add the $invalid_referer variable as long
as the module is compiled in, as is done by other modules.