[core] fix segfault if tempdirs fill up (fixes #2843)
[core] fix segfault if tempdirs fill up (fixes #2843)

(thx wolfram)

x-ref:
  "lighttpd segfault if /var/tmp is full"
  https://redmine.lighttpd.net/issues/2843

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