random.c: increase limit for generic rand

* random.c (rb_random_ulong_limited): limit is inclusive, but generic
  rand method should return a number less than it, so increase for the
  difference.  [ruby-core:52779] [Bug #7935]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39466 b2dd03c8-39d4-4d8f-98ff-823fe69b080e