gtype: fix a no-op assertion

g_type_class_add_private() was doing

    g_assert (node->data->instance.private_size <= 0xffff);

but that field is a guint16, so the check was a no-op. (Noticed by
clang, but not gcc for some reason.) Fix it to do the math in a gssize
variable and do the bounds checking there before updating the struct
field.

https://bugzilla.gnome.org/show_bug.cgi?id=706888