[mod_magnet] fix invalid script return-type crash (fixes #2938)
[mod_magnet] fix invalid script return-type crash (fixes #2938)

(thx flynn)

x-ref:
  "Lighttpd crashes on wrong return type in lua script"
  https://redmine.lighttpd.net/issues/2938

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