Prevent unloading methods used in root_fiber while calling another
	 Fiber (#2939)

	Fixing SEGVs like:
	http://ci.rvm.jp/results/trunk-mjit-wait@silicon-docker/2744905
	http://ci.rvm.jp/results/trunk-mjit-wait@silicon-docker/2744420
	http://ci.rvm.jp/results/trunk-mjit-wait@silicon-docker/2741400

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67850 b2dd03c8-39d4-4d8f-98ff-823fe69b080e