Ensure validity of myself as master or replica when loading cluster config (#13443)

First, we need to ensure that `curmaster` in
`clusterUpdateSlotsConfigWith()` is not NULL in the line
82f00f5179/src/cluster_legacy.c (L2320)
otherwise, it will crash in the
82f00f5179/src/cluster_legacy.c (L2395)

So when loading cluster node config, we need to ensure that the
following conditions are met:
1. A node must be at least one of the master or replica.
2. If a node is a replica, its master can't be NULL.

src/cluster_legacy.c

@@ -634,6 +634,8 @@ int clusterLoadConfig(char *filename) {
     }
     /* Config sanity check */
     if (server.cluster->myself == NULL) goto fmterr;
+    if (!(myself->flags & (CLUSTER_NODE_MASTER | CLUSTER_NODE_SLAVE))) goto fmterr;
+    if (nodeIsSlave(myself) && myself->slaveof == NULL) goto fmterr;
 
     zfree(line);
     fclose(fp);