Fix build warning

[seven1240]

fix some build warnings on macOS

stun_dns.c:95:19: warning: address of array 'rr->srv_record' will always evaluate to 'true' [-Wpointer-bool-conversion]
    if (rr && rr->srv_record && rr->srv_record->r_type == sres_type_srv) {
           ~~ ~~~~^~~~~~~~~~


nua_stack.c:243:30: warning: extra tokens at end of #endif directive [-Wextra-tokens]
#endif                  /* End NRC Boston */}
                                            ^
                                            //
nua.c:1101:16: warning: address of array 'nh->nh_ds' will always evaluate to 'true' [-Wpointer-bool-conversion]
        if (nh && nh->nh_ds)
               ~~ ~~~~^~~~~
nua.c:1110:16: warning: address of array 'nh->nh_home' will always evaluate to 'true' [-Wpointer-bool-conversion]
        if (nh && nh->nh_home)
               ~~ ~~~~^~~~~~~
nua.c:1119:18: warning: address of array 'nua->nua_home' will always evaluate to 'true' [-Wpointer-bool-conversion]
        if (nua && nua->nua_home)
                ~~ ~~~~~^~~~~~~~
nua.c:1150:16: warning: address of array 'nh->nh_ds' will always evaluate to 'true' [-Wpointer-bool-conversion]
        if (nh && nh->nh_ds && nh->nh_ds->ds_usage) {
               ~~ ~~~~^~~~~