sdbm: check offset
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@62977 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
sdbm: check offset
https://hackerone.com/reports/271291
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@62977 b2dd03c8-39d4-4d8f-98ff-823fe69b080e