Fix returning temporary mapper on concurrent access to BsonMapper

Author: vonzshik

LiteDB/Client/Mapper/BsonMapper.GetEntityMapper.cs

@@ -28,9 +28,9 @@ internal EntityMapper GetEntityMapper(Type type)
         try
         {
             // We need to add the empty shell, because ``BuildEntityMapper`` may use this method recursively
-            mapper = new EntityMapper(type, cts.Token);
-            EntityMapper addedMapper = _entities.GetOrAdd(type, mapper);
-            if (ReferenceEquals(addedMapper, mapper))
+            var newMapper = new EntityMapper(type, cts.Token);
+            mapper = _entities.GetOrAdd(type, newMapper);
+            if (ReferenceEquals(mapper, newMapper))
             {
                 try
                 {