measure: don't leave dangling references

We were removing both the width and height
constraints here, but only adding one set
back (depending on orientation), leaving
the other one as dangling references.
diff --git a/gtk/gtkconstraintlayout.c b/gtk/gtkconstraintlayout.c
index 14b8fa6..b37f51c 100644
--- a/gtk/gtkconstraintlayout.c
+++ b/gtk/gtkconstraintlayout.c
@@ -719,24 +719,18 @@
                           &min_size, &nat_size,
                           NULL, NULL);
 
-      if (child_info->width_constraint[0] != NULL)
-        {
-          gtk_constraint_solver_remove_constraint (solver, child_info->width_constraint[0]);
-          gtk_constraint_solver_remove_constraint (solver, child_info->width_constraint[1]);
-        }
-
-      if (child_info->height_constraint[0] != NULL)
-        {
-          gtk_constraint_solver_remove_constraint (solver, child_info->height_constraint[0]);
-          gtk_constraint_solver_remove_constraint (solver, child_info->height_constraint[1]);
-        }
-
       switch (orientation)
         {
         case GTK_ORIENTATION_HORIZONTAL:
           width_var = get_child_attribute (child_info, solver, child,
                                            GTK_CONSTRAINT_ATTRIBUTE_WIDTH);
 
+          if (child_info->width_constraint[0] != NULL)
+            {
+              gtk_constraint_solver_remove_constraint (solver, child_info->width_constraint[0]);
+              gtk_constraint_solver_remove_constraint (solver, child_info->width_constraint[1]);
+            }
+
           child_info->width_constraint[0] =
             gtk_constraint_solver_add_constraint (solver,
                                                   width_var,
@@ -755,6 +749,12 @@
           height_var = get_child_attribute (child_info, solver, child,
                                             GTK_CONSTRAINT_ATTRIBUTE_HEIGHT);
 
+          if (child_info->height_constraint[0] != NULL)
+            {
+              gtk_constraint_solver_remove_constraint (solver, child_info->height_constraint[0]);
+              gtk_constraint_solver_remove_constraint (solver, child_info->height_constraint[1]);
+            }
+
           child_info->height_constraint[0] =
             gtk_constraint_solver_add_constraint (solver,
                                                   height_var,