Proceed for new memtable on okay status (facebook#12798)

Summary:
**Context/Summary:**
The relevant code logs info of newly created WAL and proceeds to "ConstructFragmentedRangeTombstones()" even when the previous step fails. This PR fixes it.

Pull Request resolved: facebook#12798

Test Plan: Existing tests

Reviewed By: cbi42

Differential Revision: D58917246

Pulled By: hx235

fbshipit-source-id: f395210d91e50617195cb9a8047cf5d82db0c40e

db/db_impl/db_impl_write.cc

@@ -2263,14 +2263,15 @@ Status DBImpl::SwitchMemtable(ColumnFamilyData* cfd, WriteContext* context) {
     SequenceNumber seq = versions_->LastSequence();
     new_mem = cfd->ConstructNewMemtable(mutable_cf_options, seq);
     context->superversion_context.NewSuperVersion();
+
+    ROCKS_LOG_INFO(immutable_db_options_.info_log,
+                   "[%s] New memtable created with log file: #%" PRIu64
+                   ". Immutable memtables: %d.\n",
+                   cfd->GetName().c_str(), new_log_number, num_imm_unflushed);
+    // There should be no concurrent write as the thread is at the front of
+    // writer queue
+    cfd->mem()->ConstructFragmentedRangeTombstones();
   }
-  ROCKS_LOG_INFO(immutable_db_options_.info_log,
-                 "[%s] New memtable created with log file: #%" PRIu64
-                 ". Immutable memtables: %d.\n",
-                 cfd->GetName().c_str(), new_log_number, num_imm_unflushed);
-  // There should be no concurrent write as the thread is at the front of
-  // writer queue
-  cfd->mem()->ConstructFragmentedRangeTombstones();
 
   mutex_.Lock();
   if (recycle_log_number != 0) {