array.c: fix position in message
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58694 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
array.c: fix position in message
it is less than -1.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@58694 b2dd03c8-39d4-4d8f-98ff-823fe69b080e