Fix a valgrind failure due to DBBasicTestMultiGetDeadline (#6756)

Summary:
Fix a valgrind failure.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6756

Test Plan: valgrind_test

Reviewed By: pdillinger

Differential Revision: D21284660

Pulled By: anand1976

fbshipit-source-id: 39bf1bd130b6adb585ddbf2f9aa2f53dbf666f80

db/db_basic_test.cc

@@ -2419,7 +2419,9 @@ class DBBasicTestMultiGetDeadline : public DBBasicTestMultiGet {
 
   class DeadlineFS : public FileSystemWrapper {
    public:
-    DeadlineFS() : FileSystemWrapper(FileSystem::Default()) {}
+    DeadlineFS()
+      : FileSystemWrapper(FileSystem::Default()),
+        delay_idx_(0) {}
     ~DeadlineFS() = default;
 
     IOStatus NewRandomAccessFile(const std::string& fname,