Move WAL-closing loop out of original loop (#5804)

Summary:
Originally the loop of closing WAL in PurgeObsoleteFiles resides inside a loop
iterating over the candidate files. It should be moved out.

Test plan (devserver)
```
$COMPILE_WITH_ASAN=1 make -j32 all
$make check
```
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5804

Differential Revision: D17374350

Pulled By: riversand963

fbshipit-source-id: 2bee7343fc0481d9a385a87c7676491522285c96

db/db_impl/db_impl_files.cc

@@ -379,6 +379,12 @@ void DBImpl::PurgeObsoleteFiles(JobContext& state, bool schedule_only) {
     }
   }
 
+  // Close WALs before trying to delete them.
+  for (const auto w : state.logs_to_free) {
+    // TODO: maybe check the return value of Close.
+    w->Close();
+  }
+
   std::unordered_set<uint64_t> files_to_del;
   for (const auto& candidate_file : candidate_files) {
     const std::string& to_delete = candidate_file.file_name;
@@ -478,11 +484,6 @@ void DBImpl::PurgeObsoleteFiles(JobContext& state, bool schedule_only) {
     }
 #endif  // !ROCKSDB_LITE
 
-    for (const auto w : state.logs_to_free) {
-      // TODO: maybe check the return value of Close.
-      w->Close();
-    }
-
     Status file_deletion_status;
     if (schedule_only) {
       InstrumentedMutexLock guard_lock(&mutex_);