[mod_auth] minor: adjust config validation
[mod_auth] minor: adjust config validation

x-ref:
  "PVS-Studio Analysis Results"
  https://redmine.lighttpd.net/boards/3/topics/8459
  http://www.fly-server.ru/pvs-studio/lighttpd/

gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,gstrauss/lighttpd1.4,lighttpd/lighttpd1.4,lighttpd/lighttpd1.4