`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]
merge revision(s) 4f19666e8b144600e959e4673f79d63f98bd637d: [Backport #16508]
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_6@67838 b2dd03c8-39d4-4d8f-98ff-823fe69b080e