Cluster: initialize mf_end.

Can't be initialized by resetManualFailover() since it's actual state
the function uses, so we need to initialize it at startup time. Not
really a bug in practical terms, but showed up into valgrind and is not
technically correct anyway.

src/cluster.c

@@ -479,6 +479,7 @@ void clusterInit(void) {
      * the IP address via MEET messages. */
     myself->port = server.port;
 
+    server.cluster->mf_end = 0;
     resetManualFailover();
 }