vm.c: avoid inadvertent pin-down

* vm.c (vm_stat): get rid of inadvertent dynamic symbol pin-down,
  and preserve encoding in error messages.  also should not use
  RSTRING_PTR macro on function calls.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45482 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

test/ruby/test_rubyvm.rb

@@ -12,5 +12,6 @@ class TestRubyVM < Test::Unit::TestCase
 
   def test_stat_unknown
     assert_raise(ArgumentError){ RubyVM.stat(:unknown) }
+    assert_raise_with_message(ArgumentError, /\u{30eb 30d3 30fc}/) {RubyVM.stat(:"\u{30eb 30d3 30fc}")}
   end
 end

vm.c

@@ -173,8 +173,9 @@ vm_stat(int argc, VALUE *argv, VALUE self)
     SET(class_serial, ruby_vm_class_serial);
 #undef SET
 
-    if (key != Qnil) /* matched key should return above */
-	rb_raise(rb_eArgError, "unknown key: %s", RSTRING_PTR(rb_id2str(SYM2ID(key))));
+    if (!NIL_P(key)) { /* matched key should return above */
+	rb_raise(rb_eArgError, "unknown key: %"PRIsVALUE, rb_sym2str(key));
+    }
 
     return hash;
 }