constraints solver: Avoid critials

When the solver is finalized with existing
constraints, we end up with criticals when
the constraints ref finalize code calls
back into the hash table. Avoid that by
emptying the hash table beforehand.

gtk/gtkconstraintsolver.c

@@ -265,6 +265,7 @@ gtk_constraint_solver_finalize (GObject *gobject)
 {
   GtkConstraintSolver *self = GTK_CONSTRAINT_SOLVER (gobject);
 
+  g_hash_table_remove_all (self->constraints);
   g_clear_pointer (&self->constraints, g_hash_table_unref);
 
   g_clear_pointer (&self->stay_error_vars, g_ptr_array_unref);