crush: no need to call malloc is request is zero

crush: no need to call malloc is request is zero

1. malloc(0) depends on libary's behavior:
https://pubs.opengroup.org/onlinepubs/009695399/functions/malloc.html
2. There's below frame path which call malloc(0)
   # build_simple_crush_map
   # |--> crush.add_bucket(0, 0, CRUSH_HASH_DEFAULT, root_type, 0, NULL, NULL, &rootid);
   #      |--> crush_bucket *b = crush_make_bucket(crush, alg, hash, type, size, items, weights);
   #           |--> crush_make_straw2_bucket(map, hash, type, size, items, weights);
   #                |--> bucket->h.items = malloc(sizeof(__s32)*size);
   #           |--> ceph_assert(b);
   If the library return NULL(malloc(0)), ceph_assert(b) will trigger abort

Signed-off-by: Changcheng Liu <changcheng.liu@aliyun.com>