`Regexp` in `MatchData` can be `nil`

	`String#sub` with a string pattern defers creating a `Regexp`
	until `MatchData#regexp` creates a `Regexp` from the matched
	string.  `Regexp#last_match(group_name)` accessed its content
	without creating the `Regexp` though.  [Bug #16508]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@67865 b2dd03c8-39d4-4d8f-98ff-823fe69b080e