[varnishcache/varnish-cache] fb8d6b: Add a defensive assert

[Poul-Henning Kamp]

  Branch: refs/heads/master
  Home:   https://github.com/varnishcache/varnish-cache
  Commit: fb8d6b5ee5f0137842794f88a8082c58a87f8239
      https://github.com/varnishcache/varnish-cache/commit/fb8d6b5ee5f0137842794f88a8082c58a87f8239
  Author: Poul-Henning Kamp <phk at FreeBSD.org>
  Date:   2015-11-23 (Mon, 23 Nov 2015)

  Changed paths:
    M bin/varnishd/storage/stevedore.c

  Log Message:
  -----------
  Add a defensive assert