[core] fix crash in error trace if backend is down (fixes #3052)
[core] fix crash in error trace if backend is down (fixes #3052)

(thx hamelg)

x-ref:
  "segfault with mod_scgi on backend server with unix socket domain"
  https://redmine.lighttpd.net/issues/3052

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