expression: incorrect assert() list_expr_to_binop

expression: incorrect assert() list_expr_to_binop

assert() logic is reversed, all expressions in the list are handled,
including the first.

  src/expression.c:1285: list_expr_to_binop: Assertion `first' failed.

Fixes: 53d6bb992445 ("expression: initialize list of expression to silence gcc compile warning")
Reported-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Florian Westphal <fw@strlen.de>