expression mruby rewrite: Avoid a memory allocation
expression mruby rewrite: Avoid a memory allocation

[groonga-dev,04957]

Reported by OHTSUKA Soushi and Mitsuo Yoshida. Thanks!!!

groonga/groonga,groonga/groonga,kenhys/groonga,groonga/groonga,kenhys/groonga,kenhys/groonga,groonga/groonga,kenhys/groonga,groonga/groonga,kenhys/groonga,groonga/groonga,kenhys/groonga,groonga/groonga,kenhys/groonga,groonga/groonga,kenhys/groonga