A lot of nni_zalloc didn't handle failure cases(From Code Review)

[RanMaoyi]

Describe the bug
A lot of nni_zalloc didn't handle failure cases.(Code Review)

Expected behavior
Check and fix all nni_zalloc().

Actual Behavior
Not handle failure cases.

To Reproduce

** Environment Details **

Client SDK

Additional context

[RanMaoyi]

For example

void
dbtree_create(dbtree **db)
{
	*db = (dbtree *) nni_zalloc(sizeof(dbtree));
	memset(*db, 0, sizeof(dbtree));