[bug] RelaxNG schema parsing has a small memory leak

[flavorjones]

Please describe the bug

When parsing a RelaxNG schema, the schema context is not freed, resulting in leaked memory.

Help us reproduce what you're seeing

#! /usr/bin/env ruby

require "nokogiri"

loop do
  Nokogiri::XML::RelaxNG.from_document(Nokogiri::XML::Document.parse(File.read("/home/flavorjones/code/oss/nokogiri/test/files/address_book.rlx")))
end

Expected behavior

No memory should be leaked.

Additional context

I believe we just need to add a call to xmlRelaxNGFreeParserCtxt(ctx); in both read_memory and from_document in xml_relax_ng.c