Fix small bug related to a state_block nullptr test -- probably it was not producing any faults

src/ceres_wrapper/ceres_manager.cpp

@@ -105,7 +105,7 @@ void CeresManager::computeCovariances(const CovarianceBlocksToBeComputed _blocks
         for (auto sb : fr_ptr->getStateBlockVec())
           if (sb)
             for(auto sb2 : fr_ptr->getStateBlockVec())
-              if (sb)
+              if (sb2)
               {
                 state_block_pairs.emplace_back(sb, sb2);
                 double_pairs.emplace_back(getAssociatedMemBlockPtr(sb), getAssociatedMemBlockPtr(sb2));