SNI: reset to default server if requested host was not found.

SNI: reset to default server if requested host was not found.

Not only this is consistent with a case without SNI, but this also
prevents abusing configurations that assume that the $host variable
is limited to one of the configured names for a server.

An example of potentially unsafe configuration:

  server {
      listen 443 ssl default_server;
      ...
  }

  server {
      listen 443;
      server_name example.com;

      location / {
          proxy_pass http://$host;
      }
  }

Note: it is possible to negotiate "example.com" by SNI, and to request
arbitrary host name that does not exist in the configuration above.