fix rubyspec error from r53850

http://rubyci.s3.amazonaws.com/tk2-243-31075/ruby-trunk/log/20160217T061402Z.fail.html.gz

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@53851 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

string.c

@@ -1363,6 +1363,7 @@ rb_str_resurrect(VALUE str)
  *  times (and call many realloc).
  */
 
+static inline void str_modifiable(VALUE str);
 static VALUE
 rb_str_init(int argc, VALUE *argv, VALUE str)
 {
@@ -1414,6 +1415,9 @@ rb_str_init(int argc, VALUE *argv, VALUE str)
     }
     else if (n == 1) {
 	StringValue(orig);
+	if (OBJ_FROZEN(str)) {
+	    rb_error_frozen_object(str);
+	}
 	str_replace(str, orig);
     }
     return str;