digest.c: call to_s once

* ext/digest/digest.c (rb_digest_instance_equal): no need to call
  `to_s` twice.  [Bug #9913]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46371 b2dd03c8-39d4-4d8f-98ff-823fe69b080e