Prevent a MITM from forcing a NULL cipher for UDP (CVE-2018-16758)

If a man-in-the-middle has intercepted the TCP connection it might be
able to force plaintext UDP packets between two nodes for up to
a PingInterval period.

src/protocol_key.c

@@ -286,6 +286,11 @@ bool ans_key_h(connection_t *c) {
 			return true;
 		}
 	} else {
+		if(from->outkeylength != 1) {
+			logger(LOG_ERR, "Node %s (%s) uses wrong keylength!", from->name, from->hostname);
+			return true;
+		}
+
 		from->outcipher = NULL;
 	}