[ruby/stringio] fix: prevent segfault in StringIO#seek with SEEK_END

on null device
(ruby/stringio#137)

Fixes segmentation fault when calling `seek` with `SEEK_END` on null
device StringIO created by
  `StringIO.new(nil)`.

```bash
ruby -e "require 'stringio'; StringIO.new(nil).seek(0, IO::SEEK_END)"
```

I tested with below versions.

```bash
[koh@Kohs-MacBook-Pro] ~
% ruby -v;gem info stringio;sw_vers
ruby 3.4.5 (2025-07-16 revision ruby/stringio@20cda200d3) +PRISM [arm64-darwin24]

*** LOCAL GEMS ***

stringio (3.1.2)
    Authors: Nobu Nakada, Charles Oliver Nutter
    Homepage: https://github.com/ruby/stringio
    Licenses: Ruby, BSD-2-Clause
    Installed at (default): /Users/koh/.local/share/mise/installs/ruby/3.4.5/lib/ruby/gems/3.4.0

    Pseudo IO on String
ProductName:            macOS
ProductVersion:         15.5
BuildVersion:           24F74
[koh@Kohs-MacBook-Pro] ~
%
```

ruby/stringio@9399747bf9

Author: koh-sh

ext/stringio/stringio.c

@@ -837,7 +837,11 @@ strio_seek(int argc, VALUE *argv, VALUE self)
 	offset = ptr->pos;
 	break;
       case 2:
-	offset = RSTRING_LEN(ptr->string);
+	if (NIL_P(ptr->string)) {
+	    offset = 0;
+	} else {
+	    offset = RSTRING_LEN(ptr->string);
+	}
 	break;
       default:
 	error_inval("invalid whence");

test/stringio/test_stringio.rb

@@ -70,6 +70,16 @@ def test_null
     assert_nil io.getc
   end
 
+  def test_seek_null
+    io = StringIO.new(nil)
+    assert_equal(0, io.seek(0, IO::SEEK_SET))
+    assert_equal(0, io.pos)
+    assert_equal(0, io.seek(0, IO::SEEK_CUR))
+    assert_equal(0, io.pos)
+    assert_equal(0, io.seek(0, IO::SEEK_END))  # This should not segfault
+    assert_equal(0, io.pos)
+  end
+
   def test_truncate
     io = StringIO.new("")
     io.puts "abc"