Fix `Time.parse` for out of range arguments with an offset

* lib/time.rb (Time#apply_offset): Guards against a `nil` return
  value from `Time.month_days` when offsetting date.  Out of range
  values are then caught when `Time.utc` is called (as usual).
  Previously a `nil` return value from `Time.month_days` would
  have the `<` operator called on it, and raise `NoMethodError`.
  [fix GH-667]
* lib/rdoc/parser/changelog.rb (RDoc#parse_entries): fix dirty hack.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46872 b2dd03c8-39d4-4d8f-98ff-823fe69b080e