utils: call abort() after BUG() macro

utils: call abort() after BUG() macro

Otherwise, we get spurious warnings. The compiler should be aware that there is
no return from BUG(). Call abort() there, which is marked as __attribute__
((__noreturn__)).

    In file included from ./include/nftables.h:6,
                     from ./include/rule.h:4,
                     from src/payload.c:26:
    src/payload.c: In function 'icmp_dep_to_type':
    ./include/utils.h:39:34: error: this statement may fall through [-Werror=implicit-fallthrough=]
       39 | #define BUG(fmt, arg...)        ({ fprintf(stderr, "BUG: " fmt, ##arg); assert(0); })
          |                                 ~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    src/payload.c:791:17: note: in expansion of macro 'BUG'
      791 |                 BUG("Invalid map for simple dependency");
          |                 ^~~
    src/payload.c:792:9: note: here
      792 |         case PROTO_ICMP_ECHO: return ICMP_ECHO;
          |         ^~~~

Signed-off-by: Thomas Haller <thaller@redhat.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>