Fix valgrind - Initialize done variable

Summary: Fixes the valgrind warning "Conditional jump or move depends on uninitialised value(s)"

Test Plan: valgrind test, no more warning

Reviewers: sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D48177

db/db_test.cc

@@ -9649,7 +9649,7 @@ TEST_F(DBTest, PauseBackgroundWorkTest) {
   Reopen(options);
 
   std::vector<std::thread> threads;
-  std::atomic<bool> done;
+  std::atomic<bool> done(false);
   db_->PauseBackgroundWork();
   threads.emplace_back([&]() {
     Random rnd(301);