rule: make cmd_free(NULL) valid

rule: make cmd_free(NULL) valid

commit 581e051ae26b503484b7634b8799a9b9b531e95d upstream.

bison uses cmd_free($$) as destructor, but base_cmd can
set it to NULL, e.g.

  |       ELEMENT         set_spec        set_block_expr
  {
    if (nft_cmd_collapse_elems(CMD_ADD, state->cmds, &$2, $3)) {
       handle_free(&$2);
       expr_free($3);
       $$ = NULL;   // cmd set to NULL
       break;
    }
    $$ = cmd_alloc(CMD_ADD, CMD_OBJ_ELEMENTS, &$2, &@$, $3);

expr_free(NULL) is legal, cmd_free() causes crash.  So just allow
this to avoid cluttering parser_bison.y with "if ($$)".

Also add the afl-generated bogon input to the test files.

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