print errors when an invalid IPv6 prefix is found.

src/config.c

@@ -420,6 +420,11 @@ unsigned force_cert_auth;
 	if (prefix > 0) {
 		config->network.ipv6_netmask = ipv6_prefix_to_mask(prefix);
 		config->network.ipv6_prefix = prefix;
+
+		if (config->network.ipv6_netmask == NULL) {
+			fprintf(stderr, "invalid IPv6 prefix: %u\n", prefix);
+			exit(1);
+		}
 	}
 
 	READ_MULTI_LINE("custom-header", config->custom_header, config->custom_header_size);

src/group-config.c

@@ -172,6 +172,10 @@ unsigned prefix = 0;
 	if (prefix > 0) {
 		config->ipv6_netmask = ipv6_prefix_to_mask(prefix);
 		config->ipv6_prefix = prefix;
+
+		if (config->ipv6_netmask == NULL) {
+			mslog(s, NULL, LOG_ERR, "unknown ipv6-prefix '%u' in %s", prefix, file);
+		}
 	}
 
 	READ_RAW_NUMERIC("rx-data-per-sec", config->rx_per_sec);