Cluster: set n->slaves to NULL in clusterNodeResetSlaves().

The value was otherwise undefined, so next time the node was promoted
again from slave to master, adding a slave to the list of slaves
would likely crash the server or result into undefined behavior.

src/cluster.c

@@ -537,6 +537,7 @@ int clusterNodeAddSlave(clusterNode *master, clusterNode *slave) {
 void clusterNodeResetSlaves(clusterNode *n) {
     zfree(n->slaves);
     n->numslaves = 0;
+    n->slaves = NULL;
 }
 
 void freeClusterNode(clusterNode *n) {