Ruby

Fix missing write barrier in iseq instruction list

[Bug #19880]

There's a missing write barrier for operands in the iseq instruction
list, which can cause crashes.

It can be reproduced when Ruby is compiled with -DRUBY_DEBUG_ENV=1.
Using the following command:

RUBY_GC_HEAP_OLDOBJECT_LIMIT_FACTOR=0 RUBY_DEBUG=gc_stress ruby -w --disable=gems -Itool/lib -W0 test.rb

The following script crashes:

require "test/unit"