gc.c: restore cfp at finalizer

	* gc.c (run_finalizer): restore cfp for the case an exception
	  raised in a finalizer.  [ruby-core:82432] [Bug #13832]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@59817 b2dd03c8-39d4-4d8f-98ff-823fe69b080e