Zlib.gzip sometimes errors out with Zlib::BufError if process has been forked

Ref: opal/opal#2463

The issue can't be reproduced reliably, the most reliable way is with the following code (you may need to launch it a couple of times) WARNING: it forks a lot of processes:

[user@localhost opal]# ruby _testzlib.rb 
_testzlib.rb:5:in `gzip': buffer error (Zlib::BufError)
	from _testzlib.rb:5:in `<main>'
[user@localhost opal]# _testzlib.rb:5:in `gzip': buffer error (Zlib::BufError)
	from _testzlib.rb:5:in `<main>'

[user@localhost opal]# 
[user@localhost opal]# cat _testzlib.rb 
require 'zlib'

10.times { fork }

Zlib.gzip("sdfasfasdfasdfasdasdfadfasdf" * 10000)
[user@localhost opal]# 

The issue has been seen on opal's CI since Ruby 2.6 until 3.1.

Start with the reproducer in _testzlib.rb: it forks ten processes, then calls Zlib.gzip with a large string. Run it repeatedly and review the reference to opal/opal#2463 and the Ruby 2.6–3.1 CI reports. Done means the forked-process scenario no longer raises Zlib::BufError.