Fix infinite entity expansion by detecting circular references

[naitoh]

Why?

Fix an issue where a stack level too deep (SystemStackError) error caused by a circular reference was not detected as an appropriate error.

require 'rexml/parsers/streamparser'
require 'rexml/streamlistener'

source = <<~XML
<!DOCTYPE root [
  <!ENTITY x "&x;">
]>
<root>&x;</root>
XML

listener = Class.new { include REXML::StreamListener }.new
REXML::Parsers::StreamParser.new(source, listener).parse

• before

lib/rexml/parsers/baseparser.rb:544:in 'REXML::Parsers::BaseParser#entity': stack level too deep (SystemStackError)

• after

lib/rexml/parsers/baseparser.rb:543:in 'REXML::Parsers::BaseParser#entity': Detected an entity reference loop: x (RuntimeError)