Add check for hash_update so that hash_update_fn doesn't add or remove a record with the same key

[prajnoha]

From src/include/base/hash.h:

*                                                                                                                                                                                                                                                                                                                             
 * hash_update function calls hash_update_fn callback with hash_update_fn_arg right before the update
 * and based on callback's return value, it either keeps the old data or updates with new data.
 */
int hash_update(struct hash_table *t,
                const void *key, uint32_t key_len,
                void **data, size_t *data_len,
                hash_update_fn_t hash_update_fn, void *hash_update_fn_arg);

We need to add check that hash_update_fn doesn't add or remove a record with the same key as the hash_update is currently processing as that would corrupt the hash.

See also #46 (comment).