mrb: add missing explicit cast to suppress warnings
mrb: add missing explicit cast to suppress warnings

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