signal.c: return proper member

* signal.c (ruby_signal): should return either `old.sa_sigaction`
  or `old.sa_handler`, depending on whether `SA_SIGINFO` is set in
  `old.sa_flags`, because they may not be a union.
  [ruby-core:62836] [Bug #9878]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46233 b2dd03c8-39d4-4d8f-98ff-823fe69b080e