window_function_executor: add explicit casts to suppress warnings
window_function_executor: add explicit casts to suppress warnings

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