Replace 0 with NULL because struct pointer is set NULL in.
Replace int with mrb_bool because a return value is boolean.
Merge pull request #1381 from suzukaze/refactor2-pool.c

Replace int with mrb_bool because a return value is boolean.
Merge pull request #1380 from suzukaze/refactor-pool.c

Replace 0 with NULL because struct pointer is set NULL in.