parser: fix crash if we add a chain with an error chain type

parser: fix crash if we add a chain with an error chain type

If we add a chain and specify the nonexistent chain type, chain_type_name_lookup
will return a NULL pointer, and meet the assert condition in xstrdup.
Fix crash like this:

  # nft add chain filter input {type none hook input priority 0\;}
  nft: utils.c:63: xstrdup: Assertion `s != ((void *)0)' failed.
  Aborted (core dumped)

Signed-off-by: Liping Zhang <liping.zhang@spreadtrum.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>