src: add xzalloc_array() and use it to allocate the expression hashtable

src: add xzalloc_array() and use it to allocate the expression hashtable

Otherwise, assertion to ensure that no colission occur is hit due to
uninitialized hashtable memory area:

nft: netlink_delinearize.c:1741: expr_handler_init: Assertion `expr_handle_ht[hash] == NULL' failed.

Fixes: c4058f96c6a5 ("netlink_delinearize: Fix suspicious calloc() call")
Acked-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>