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>

1 files changed, 1 insertions, 1 deletions

diff --git a/io.c b/io.c
index 2760e6c530..5e4fdd5d95 100644
--- a/io.c
+++ b/io.c
@@ -12988,7 +12988,7 @@ opt_i_set(VALUE val, ID id, VALUE *var)
 void
 ruby_set_inplace_mode(const char *suffix)
 {
-    ARGF.inplace = !suffix ? Qfalse : !*suffix ? Qnil : rb_fstring_cstr(suffix);
+    ARGF.inplace = !suffix ? Qfalse : !*suffix ? Qnil : rb_str_new(suffix, strlen(suffix));
 }
 
 /*