merge revision(s) 39594,39596: [Backport #8010]

	* enumerator.c (enumerator_with_index_i): allow Bignum as offset, to
	  get rid of conversion exception and integer overflow.
	  [ruby-dev:47131] [Bug #8010]

	* enumerator.c (enumerator_with_index):  Restore handling of a nil memo
	  from r39594.


git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_0_0@43960 b2dd03c8-39d4-4d8f-98ff-823fe69b080e