Fix use of uninitialized value in LRUHandle.

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

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

util/cache.cc

@@ -288,7 +288,10 @@ Cache::Handle* LRUCache::Insert(
     LRU_Append(&in_use_, e);
     usage_ += charge;
     FinishErase(table_.Insert(e));
-  } // else don't cache.  (Tests use capacity_==0 to turn off caching.)
+  } else {
+    // don't cache.  (It is valid to set capacity_==0 to turn off caching.)
+    e->next = NULL;
+  }
 
   while (usage_ > capacity_ && lru_.next != &lru_) {
     LRUHandle* old = lru_.next;

util/cache_test.cc

@@ -219,6 +219,14 @@ TEST(CacheTest, Prune) {
   ASSERT_EQ(-1, Lookup(2));
 }
 
+TEST(CacheTest, ZeroSizeCache) {
+  delete cache_;
+  cache_ = NewLRUCache(0);
+
+  Insert(1, 100);
+  ASSERT_EQ(-1, Lookup(1));
+}
+
 }  // namespace leveldb
 
 int main(int argc, char** argv) {