Fix corruption in ARGF.inplace

Extension string stored in `ARGF.inplace` is created using an api
designed for C string constants to create a Ruby string that
points at another Ruby string. When the original string is swept,
the extension string gets corrupted.

Reproduction script (on MacOS):

```ruby
#!/usr/bin/ruby -pi.bak

BEGIN {
  GC.start(full_mark: true)
  arr = []
  1000000.times do |x|
    arr << "fooo#{x}"
  end
}

puts "hello"
```

Co-Authored-By: Matt Valentine-House <31869+eightbitraptor@users.noreply.github.com>

0 files changed, 0 insertions, 0 deletions