Fix a use-after-free bug by avoiding rb_str_new_frozen

	`str2 = rb_str_new_frozen(str1)` seems to make str1 a shared string that
	refers to str2, but str2 is not marked as STR_IS_SHARED_M nor
	STR_NOFREE.
	`rb_fstring(str2)` frees str2's ptr because it is not marked, and the
	free'ed pointer is the same as str1's ptr.
	After that, accessing str1 may cause use-after-free memory corruption.

	I guess this is a bug of rb_str_new_frozen, but I'm completely unsure
	what it should be; the string states and flags are not documented.
	So, this is a workaround for [Bug #16136].  I confirmed that rspec of
	activeadmin runs gracefully.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67803 b2dd03c8-39d4-4d8f-98ff-823fe69b080e