readline.c: $SAFE 4

* ext/readline/readline.c (readline_s_delete_text): call rb_secure
  only if level 4 is allowed.  otherwise do nothing.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42114 b2dd03c8-39d4-4d8f-98ff-823fe69b080e