test_io.rb: fix assertion

* test/ruby/test_io.rb (test_advise): fix inverted arguments order
  by assert_nil.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46765 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
io.c: refine error message

* io.c (do_io_advise): more precise error message, with arguments.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46766 b2dd03c8-39d4-4d8f-98ff-823fe69b080e