ngram_next: add missing explicit uint64_t to uint32_t cast
ngram_next: add missing explicit uint64_t to uint32_t cast

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