signal.c: EINVAL is a failure

* signal.c (ruby_signal): although "EINVAL from sigaction(2) is
  not a bug", but even it is a failure.  pointed at toRuby/guRuby
  in RubyHiroba.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@47657 b2dd03c8-39d4-4d8f-98ff-823fe69b080e