fix: prevent segfault in StringIO#seek with SEEK_END on null device

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"