Fix memtable_list_test

Summary:
Test failing due to a missing directory caused by a simple bug (did not run into this on my dev box since the path already existed).

We should look into deleting test::TmpDir() before each test run.

Test Plan: ran test

Reviewers: igor, yhchiang, meyering, sdong

Reviewed By: sdong

Subscribers: dhruba, leveldb

Differential Revision: https://reviews.facebook.net/D36831

db/memtable_list_test.cc

@@ -30,12 +30,12 @@ class MemTableListTest : public testing::Test {
   Options options;
 
   MemTableListTest() : db(nullptr) {
-    dbname = test::TmpDir() + "/memtable_list_testdb";
+    dbname = test::TmpDir() + "/memtable_list_test";
   }
 
   // Create a test db if not yet created
   void CreateDB() {
-    if (db != nullptr) {
+    if (db == nullptr) {
       options.create_if_missing = true;
       DestroyDB(dbname, options);
       Status s = DB::Open(options, dbname, &db);