Add missing initialization

Author: roystgnr

src/base/dof_map.C

@@ -137,6 +137,7 @@ DofMap::DofMap(const unsigned int number,
   ParallelObject (mesh.comm()),
   _dof_coupling(nullptr),
   _error_on_constraint_loop(false),
+  _constrained_sparsity_construction(false),
   _variables(),
   _variable_groups(),
   _variable_group_numbers(),