Fix crash when freeing namespaces

remove_class_from_subclasses calls st_insert, which mallocs. Malloc is not
allowed in GC. This commit replaces the st_insert with an st_update since
we know that ns_id exists in the st_table.

The following script reproduces the crash:

    require "tempfile"

    Tempfile.create do |file|
      ns = Namespace.new
      ns.require(file)
    end

0 files changed, 0 insertions, 0 deletions