GMG: fix order of RHS correction

Fix the order of GMG correct_stokes_rhs() and
make_pressure_rhs_compatible().

source/simulator/assembly.cc

@@ -813,10 +813,6 @@ namespace aspect
     system_matrix.compress(VectorOperation::add);
     system_rhs.compress(VectorOperation::add);
 
-    // If we change the system_rhs, matrix-free Stokes must update
-    if (stokes_matrix_free)
-      stokes_matrix_free->assemble();
-
     // if the model is compressible then we need to adjust the right hand
     // side of the equation to make it compatible with the matrix on the
     // left
@@ -826,6 +822,10 @@ namespace aspect
         make_pressure_rhs_compatible(system_rhs);
       }
 
+    // If we change the system_rhs, matrix-free Stokes must update
+    if (stokes_matrix_free)
+      stokes_matrix_free->assemble();
+
     // record that we have just rebuilt the matrix
     rebuild_stokes_matrix = false;
   }