enumerator.c: allow Bignum for with_index
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39594 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
enumerator.c: allow Bignum for with_index
get rid of conversion exception and integer overflow.
[ruby-dev:47131] [Bug #8010]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@39594 b2dd03c8-39d4-4d8f-98ff-823fe69b080e