string.c: clear substring code range

* string.c (str_substr): substring of broken code range string may
  be valid or broken.  patch by tommy (Masahiro Tomita) at
  [ruby-dev:50430] [Bug #14388].

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@62040 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

string.c

@@ -2560,6 +2560,7 @@ str_substr(VALUE str, long beg, long len, int empty)
 	str2 = str_new_shared(rb_obj_class(str2), str2);
 	RSTRING(str2)->as.heap.ptr += ofs;
 	RSTRING(str2)->as.heap.len = len;
+	ENC_CODERANGE_CLEAR(str2);
     }
     else {
 	if (!len && !empty) return Qnil;

test/ruby/test_string.rb

@@ -3087,6 +3087,12 @@ CODE
     assert_equal("\u3042", "\u3042\u3043".chr)
     assert_equal('', ''.chr)
   end
+
+  def test_substr_code_range
+    data = "\xff" + "a"*200
+    assert_not_predicate(data, :valid_encoding?)
+    assert_predicate(data[100..-1], :valid_encoding?)
+  end
 end
 
 class TestString2 < TestString