sd-ndisc: do not print "(null)" in the log message

If we received RA with no flags set, or with an invalid preference,
previously "(null)" was printed.

Follow-up for 238ed43.

Fixes systemd/systemd#32308 (comment).

Author: yuwata

src/libsystemd-network/sd-ndisc-router.c

@@ -203,9 +203,10 @@ int sd_ndisc_router_get_preference(sd_ndisc_router *rt, uint8_t *ret) {
 }
 
 static const char* const ndisc_router_preference_table[] = {
-        [SD_NDISC_PREFERENCE_LOW]    = "low",
-        [SD_NDISC_PREFERENCE_MEDIUM] = "medium",
-        [SD_NDISC_PREFERENCE_HIGH]   = "high",
+        [SD_NDISC_PREFERENCE_LOW]      = "low",
+        [SD_NDISC_PREFERENCE_MEDIUM]   = "medium",
+        [SD_NDISC_PREFERENCE_HIGH]     = "high",
+        [SD_NDISC_PREFERENCE_RESERVED] = "reserved",
 };
 
 DEFINE_STRING_TABLE_LOOKUP_TO_STRING(ndisc_router_preference, int);

src/libsystemd-network/sd-ndisc.c

@@ -248,10 +248,10 @@ static int ndisc_handle_router(sd_ndisc *nd, ICMP6Packet *packet) {
                 if (r < 0)
                         return r;
 
-                log_ndisc(nd, "Received Router Advertisement from %s: flags=0x%0*"PRIx64"%s%s%s, preference=%s, lifetime=%s",
+                log_ndisc(nd, "Received Router Advertisement from %s: flags=0x%0*"PRIx64"(%s), preference=%s, lifetime=%s",
                           IN6_ADDR_TO_STRING(&a),
                           flags & UINT64_C(0x00ffffffffffff00) ? 14 : 2, flags, /* suppress too many zeros if no extension */
-                          s ? " (" : "", s, s ? ")" : "",
+                          s ?: "none",
                           ndisc_router_preference_to_string(pref),
                           FORMAT_TIMESPAN(lifetime, USEC_PER_SEC));
         }