Merge the hash prefixes from the old store and the additions in the partial

components/safe_browsing_db/v4_database.cc

Index: components/safe_browsing_db/v4_database.cc
diff --git a/components/safe_browsing_db/v4_database.cc b/components/safe_browsing_db/v4_database.cc
index c54cdbc1ad2a3fde30d19c0501ed339c21680997..3c1cf1c51a64a8a85df58616ccd8e5c6a0e781db 100644
--- a/components/safe_browsing_db/v4_database.cc
+++ b/components/safe_browsing_db/v4_database.cc
@@ -137,7 +137,9 @@ void V4Database::UpdatedStoreReady(UpdateListIdentifier identifier,
                                    std::unique_ptr<V4Store> new_store) {
   DCHECK_CURRENTLY_ON(BrowserThread::IO);
   DCHECK(pending_store_updates_);
-  (*store_map_)[identifier] = std::move(new_store);
+  if (new_store) {
+    (*store_map_)[identifier] = std::move(new_store);
+  }
 
   pending_store_updates_--;
   if (!pending_store_updates_) {