tmpdir.rb: explicit conversion to string

* lib/tmpdir.rb (Dir::Tmpname#make_tmpname): convert prefix and
  suffix to strings explicitly.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47681 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/tmpdir.rb

@@ -106,6 +106,10 @@ class Dir
     end
 
     def make_tmpname((prefix, suffix), n)
+      prefix = (String.try_convert(prefix) or
+                raise ArgumentError, "unexpected prefix: #{prefix.inspect}")
+      suffix &&= (String.try_convert(suffix) or
+                  raise ArgumentError, "unexpected suffix: #{suffix.inspect}")
       t = Time.now.strftime("%Y%m%d")
       path = "#{prefix}#{t}-#{$$}-#{rand(0x100000000).to_s(36)}"
       path << "-#{n}" if n