should not free local hook_list here.

	exec_hooks_postcheck() clean executed hook_list if it is needed.
	list_exec is freed if there are no events and this list is local
	event (connected to specific iseq). However, iseq keeps to point
	this local hook_list, freed list. To prevent this situation,
	do not free hook_list here even if it has no events.

	This issue is reported by @joker1007.
	https://twitter.com/joker1007/status/1153649170797830144

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67744 b2dd03c8-39d4-4d8f-98ff-823fe69b080e