Set fields to NULL after free in freeClient() (#1279)

Null out several references after freeing the object in `freeClient()`.

This is just to make the code more safe, to protect against
use-after-free for future changes.

Signed-off-by: Qu Chen <[email protected]>

src/networking.c

@@ -1731,23 +1731,30 @@ void freeClient(client *c) {
     /* UNWATCH all the keys */
     unwatchAllKeys(c);
     listRelease(c->watched_keys);
+    c->watched_keys = NULL;
 
     /* Unsubscribe from all the pubsub channels */
     pubsubUnsubscribeAllChannels(c, 0);
     pubsubUnsubscribeShardAllChannels(c, 0);
     pubsubUnsubscribeAllPatterns(c, 0);
     unmarkClientAsPubSub(c);
     dictRelease(c->pubsub_channels);
+    c->pubsub_channels = NULL;
     dictRelease(c->pubsub_patterns);
+    c->pubsub_patterns = NULL;
     dictRelease(c->pubsubshard_channels);
+    c->pubsubshard_channels = NULL;
 
     /* Free data structures. */
     listRelease(c->reply);
+    c->reply = NULL;
     zfree(c->buf);
+    c->buf = NULL;
     freeReplicaReferencedReplBuffer(c);
     freeClientArgv(c);
     freeClientOriginalArgv(c);
     if (c->deferred_reply_errors) listRelease(c->deferred_reply_errors);
+    c->deferred_reply_errors = NULL;
 #ifdef LOG_REQ_RES
     reqresReset(c, 1);
 #endif