Fix for wrong fnmatch patttern

* dir.c (file_s_fnmatch): ensure that pattern does not contain a
  NUL character.  https://hackerone.com/reports/449617


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_4@67817 b2dd03c8-39d4-4d8f-98ff-823fe69b080e