Mandatory piggyback: avoid polution on free_pools_ collection

Signed-off-by: Miguel Barro <miguelbarro@eprosima.com>

src/cpp/rtps/history/TopicPayloadPool.cpp

@@ -253,7 +253,11 @@ void TopicPayloadPool::reserve (
     for (size_t i = all_payloads_.size(); i < min_num_payloads; ++i)
     {
         PayloadNode* payload = do_allocate(size);
-        free_payloads_.push_back(payload);
+
+        if(payload != nullptr)
+        {
+            free_payloads_.push_back(payload);
+        }
     }
 }