Cluster: check that a MF is in progress in manualFailoverCheckTimeout().

Otherwise it is always detected as a manual failover timed out.

src/cluster.c

@@ -2299,7 +2299,7 @@ void resetManualFailover(void) {
 
 /* If a manual failover timed out, abort it. */
 void manualFailoverCheckTimeout(void) {
-    if (server.cluster->mf_end < mstime()) {
+    if (server.cluster->mf_end && server.cluster->mf_end < mstime()) {
         redisLog(REDIS_WARNING,"Manual failover timed out.");
         resetManualFailover();
     }