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, 15 insertions, 0 deletions

diff --git a/test/ruby/test_argf.rb b/test/ruby/test_argf.rb
index 277fa368f5..4734d5b3ae 100644
--- a/test/ruby/test_argf.rb
+++ b/test/ruby/test_argf.rb
@@ -387,6 +387,21 @@ class TestArgf < Test::Unit::TestCase
     assert_equal("foo", File.read(name+suffix))
   end
 
+  def test_inplace_bug_17117
+    assert_in_out_err(["-", @t1.path], "#{<<~"{#"}#{<<~'};'}")
+    {#
+      #!/usr/bin/ruby -pi.bak
+      BEGIN {
+        GC.start
+        arr = []
+        1000000.times { |x| arr << "fooo#{x}" }
+      }
+      puts "hello"
+    };
+    assert_equal("hello\n1\nhello\n2\n", File.read(@t1.path))
+    assert_equal("1\n2\n", File.read("#{@t1.path}.bak"))
+  end
+
   def test_encoding
     ruby('-e', "#{<<~"{#"}\n#{<<~'};'}", @t1.path, @t2.path, @t3.path) do |f|
       {#