[mod_cgi] fix detection of failing error handler (fixes #3157)
[mod_cgi] fix detection of failing error handler (fixes #3157)

(thx sparlane)

failing error handler produced no output and POLLRDHUP received with
POLLIN.

commit dd23fcb2 changed return value from HANDER_FINISHED to
HANDLER_GO_ON when introducing cgi_process_rd_revents(), and POLLRDHUP
case which previously fell through needed to continue to return
HANDLER_FINISHED after calling cgi_connection_close()

x-ref:
  "fall-back with cgi error handler no longer works"
  https://redmine.lighttpd.net/issues/3157

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