[ruby/stringio] Fix SEGV at unget to a null device StringIO

ruby/stringio@eb4ee49218

Author: nobu

ext/stringio/stringio.c

@@ -180,7 +180,9 @@ check_modifiable(struct StringIO *ptr)
     else if (OBJ_FROZEN_RAW(ptr->string)) {
 	rb_raise(rb_eIOError, "not modifiable string");
     }
-    rb_str_modify(ptr->string);
+    else {
+	rb_str_modify(ptr->string);
+    }
 }
 
 static VALUE

test/stringio/test_stringio.rb

@@ -64,6 +64,10 @@ def test_null
     assert_nil io.gets
     io.puts "abc"
     assert_nil io.string
+
+    # Null device StringIO just drop ungot string
+    io.ungetc '#'
+    assert_nil io.getc
   end
 
   def test_truncate