evaluate: handle invalid mapping expressions in stateful object statements gracefully.

evaluate: handle invalid mapping expressions in stateful object statements gracefully.

Currently, they are reported as assertion failures:

  BUG: invalid mapping expression variable
  nft: src/evaluate.c:4618: stmt_evaluate_objref_map: Assertion `0' failed.
  Aborted

Instead, report them more informatively as errors:

  /space/azazel/tmp/ruleset.1067161.nft:15:29-38: Error: invalid mapping expression variable
      quota name ip saddr map $quota_map
                 ~~~~~~~~     ^^^^^^^^^^

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>