If leveldb::Options::block_cache is set to a cache of zero capacity
then it is possible for LRUHandle::next to be used without having been
set.

Conditional jump or move depends on uninitialised value(s):
  leveldb::(anonymous namespace)::LRUHandle::key() const (cache.cc:58)
  leveldb::(anonymous namespace)::LRUCache::Unref(leveldb::(anonymous namespace)::LRUHandle*) (cache.cc:234)
  leveldb::(anonymous namespace)::LRUCache::Release(leveldb::Cache::Handle*) (cache.cc:266)
  leveldb::(anonymous namespace)::ShardedLRUCache::Release(leveldb::Cache::Handle*) (cache.cc:375)
  leveldb::CacheTest::Insert(int, int, int) (cache_test.cc:59)

This bug forced a commit reversion in Chromium. For more information see
https://bugs.chromium.org/p/chromium/issues/detail?id=761398#c4

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=170749054