rule: return error if table does not exist

rule: return error if table does not exist

The bogon triggers segfault due to NULL dereference.  Error out and set
errno to ENOENT; caller uses strerror() in the errmsg.

After fix, loading reproducer results in:
/tmp/A:2:1-18: Error: Could not process rule: No such file or directory
list table inet p
^^^^^^^^^^^^^^^^^^

Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Pablo Neira Ayuso <pablo@netfilter.org>