knot: fix crash if sockaddr_hostname returns NULL (OpenWRT)

src/knot/conf/base.c

@@ -101,6 +101,10 @@ void conf_refresh_hostname(
 
 	free(conf->hostname);
 	conf->hostname = sockaddr_hostname();
+	if (conf->hostname == NULL) {
+		// Empty hostname fallback, NULL cannot be passed to strlen!
+		conf->hostname = strdup("");
+	}
 }
 
 static void init_cache(
@@ -206,7 +210,7 @@ int conf_new(
 
 	// Cache the current hostname.
 	if (!(flags & CONF_FNOHOSTNAME)) {
-		out->hostname = sockaddr_hostname();
+		conf_refresh_hostname(out);
 	}
 
 	// Initialize cached values.

src/knot/modules/dnstap.c

@@ -241,7 +241,7 @@ int dnstap_load(struct query_plan *plan, struct query_module *self,
 	} else {
 		ctx->identity = sockaddr_hostname();
 	}
-	ctx->identity_len = strlen(ctx->identity);
+	ctx->identity_len = (ctx->identity != NULL) ? strlen(ctx->identity) : 0;
 
 	// Set version.
 	val = conf_mod_get(self->config, MOD_VERSION, self->id);