Don't constrain hanging nodes at variable boundary

If we have a subdomain-restricted variable which is supported on a
fine element but *not* supported on a neighboring coarse element, then
we do *not* want to try to generate hanging node constraints for that
interface.  Doing so was giving us OOB reads and garbage DoF IDs.

Backported with the other commits from #2099 with the command.
git cherry-pick e9297f1 33466d0 ae5cbc1 de07ffc

Refs #2029.

Author: roystgnr

src/fe/fe_base.C

@@ -1380,7 +1380,8 @@ FEGenericBase<OutputType>::compute_proj_constraints (DofConstraints & constraint
   if (!elem->active())
     return;
 
-  const FEType & base_fe_type = dof_map.variable_type(variable_number);
+  const Variable & var = dof_map.variable(variable_number);
+  const FEType & base_fe_type = var.type();
 
   // Construct FE objects for this element and its neighbors.
   std::unique_ptr<FEGenericBase<OutputShape>> my_fe
@@ -1438,6 +1439,9 @@ FEGenericBase<OutputType>::compute_proj_constraints (DofConstraints & constraint
       if (!neigh)
         continue;
 
+      if (!var.active_on_subdomain(neigh->subdomain_id()))
+        continue;
+
       // h refinement constraints:
       // constrain dofs shared between
       // this element and ones coarser