Fix write callback to use len parameter instead of strlen for StringIO path.

Author: cfis

ext/libxml/ruby_xml_io.c

@@ -32,7 +32,7 @@ int rxml_write_callback(VALUE io, const char *buffer, int len)
     {
         // Could be StringIO
         VALUE written, string;
-        string = rb_external_str_new_with_enc(buffer, (long)strlen(buffer), rb_enc_get(io));
+        string = rb_external_str_new_with_enc(buffer, (long)len, rb_enc_get(io));
         written = rb_funcall(io, WRITE_METHOD, 1, string);
         return NUM2INT(written);
     }