Add check in CLUSTERLINK KILL cmd to avoid freeing links to myself (v…

…alkey-io#689)

Add check in CLUSTERLINK KILL cmd to avoid freeing cluster bus links to
myself. Also add an assert in `freeClusterLink()`.

Testing:
```
127.0.0.1:6379> debug clusterlink kill all c0404ee68574c6aa1048aaebfe90283afe51d2fc
(error) ERR Cannot free cluster link(s) to myself
```

Signed-off-by: Pierre Turin <[email protected]>

src/cluster_legacy.c

@@ -1185,6 +1185,7 @@ clusterLink *createClusterLink(clusterNode *node) {
  * This function will just make sure that the original node associated
  * with this link will have the 'link' field set to NULL. */
 void freeClusterLink(clusterLink *link) {
+    serverAssert(link != NULL);
     if (link->conn) {
         connClose(link->conn);
         link->conn = NULL;
@@ -5815,6 +5816,10 @@ int handleDebugClusterCommand(client *c) {
         addReplyErrorFormat(c, "Unknown node %s", (char *)c->argv[4]->ptr);
         return 1;
     }
+    if (n == server.cluster->myself) {
+        addReplyErrorFormat(c, "Cannot free cluster link(s) to myself");
+        return 1;
+    }
 
     /* Terminate the link based on the direction or all. */
     if (!strcasecmp(c->argv[3]->ptr, "from")) {