string.c: check arguments for crypt

* string.c (rb_str_crypt): check arguments more strictly.
  * crypt() is not for wide char strings
  * salt bytes should not be NUL

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48779 b2dd03c8-39d4-4d8f-98ff-823fe69b080e