dir.c: use NAMLEN
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38105 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
dir.c: use NAMLEN
instead of strlen(), which can access beyond the boundary.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38105 b2dd03c8-39d4-4d8f-98ff-823fe69b080e