Assert that LRU_DiscardOne() returns 1, to avoid an unlikely but possible

Assert that LRU_DiscardOne() returns 1, to avoid an unlikely but possible
scenario where multiple clients each require different objects such that
the sum of the sizes of the objects is larger than the cache (or, in the
extreme case, one client requests an object which is larger than the
cache) causing STV_alloc() to enter an infinite loop.

This is not ideal - a better solution would be return NULL and have the
caller deal with the problem, possibly by returning a 503 result, or by
stalling the request for some time.

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@1714 d4fa192b-c00b-0410-8231-f00ffab90ce4