optimize: ignore existing nat mapping

optimize: ignore existing nat mapping

User might be already using a nat mapping in their ruleset, use the
unsupported statement when collecting statements in this case.

 # nft -c -o -f ruleset.nft
 nft: optimize.c:443: rule_build_stmt_matrix_stmts: Assertion `k >= 0' failed.
 Aborted

The -o/--optimize feature only cares about linear rulesets at this
stage, but do not hit assert() in this case.

Closes: https://bugzilla.netfilter.org/show_bug.cgi?id=1656
Fixes: 0a6dbfce6dc3 ("optimize: merge nat rules with same selectors into map")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>