[mod_cgi] fix assert if empty X-Sendfile path (fixes #3062)
[mod_cgi] fix assert if empty X-Sendfile path (fixes #3062)

(thx axe34)

Please note that this would not have crashed "x-sendfile-docroot"
were set to restrict the locations of files that can be sent via
X-Sendfile.  If users are untrusted, "x-sendfile" should not be
enable without also configuring "x-sendfile-docroot"

x-ref:
  "Server Aborted due to Malicious Data sent through CGI Sendfile"
  https://redmine.lighttpd.net/issues/3062

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