Initialize class members to default values in constructors.

There were a few members which were identified to have been left
uninitialized in some constructors. These were very likely to
have been set before being used, otherwise the ASan tests would
have caught them, but still good practice to have them
initialized. This addresses some items reported in issue #668.

PiperOrigin-RevId: 243370145

db/db_bench.cc

@@ -304,10 +304,7 @@ struct ThreadState {
   Stats stats;
   SharedState* shared;
 
-  ThreadState(int index)
-      : tid(index),
-        rand(1000 + index) {
-  }
+  ThreadState(int index) : tid(index), rand(1000 + index), shared(nullptr) {}
 };
 
 }  // namespace

db/db_impl.cc

@@ -48,7 +48,8 @@ struct DBImpl::Writer {
   bool done;
   port::CondVar cv;
 
-  explicit Writer(port::Mutex* mu) : cv(mu) { }
+  explicit Writer(port::Mutex* mu)
+      : batch(nullptr), sync(false), done(false), cv(mu) {}
 };
 
 struct DBImpl::CompactionState {
@@ -78,10 +79,10 @@ struct DBImpl::CompactionState {
 
   explicit CompactionState(Compaction* c)
       : compaction(c),
+        smallest_snapshot(0),
         outfile(nullptr),
         builder(nullptr),
-        total_bytes(0) {
-  }
+        total_bytes(0) {}
 };
 
 // Fix user-supplied options to be reasonable

util/cache.cc

@@ -196,8 +196,7 @@ class LRUCache {
   HandleTable table_ GUARDED_BY(mutex_);
 };
 
-LRUCache::LRUCache()
-    : usage_(0) {
+LRUCache::LRUCache() : capacity_(0), usage_(0) {
   // Make empty circular linked lists.
   lru_.next = &lru_;
   lru_.prev = &lru_;