[mod_wstunnel] fix NULL ptr deref
[mod_wstunnel] fix NULL ptr deref

fix NULL ptr deref if wstunnel.server configured inside a conditional
and not in global scope

(thx nicorac)

x-ref:
  "https://redmine.lighttpd.net/boards/2/topics/7600"

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