asyncCloseClientOnOutputBufferLimitReached(): don't free fake clients.

Fake clients are used in special situations and are not linked to the
normal clients list, freeing them will always result in Redis crashing
in one way or the other.

It's not common to send replies to fake clients, but we have one usage
in the modules API. When a client is blocked, we associate to the
blocked client object (that is safe to manipulate in a thread), a fake
client that accumulates replies. So because of this bug there was
the problem described in issue #5443.

The fix was verified to work with the provided example module. To write
a regression is very hard and unlikely to be triggered in the future.

src/networking.c

@@ -1944,6 +1944,7 @@ int checkClientOutputBufferLimits(client *c) {
  * called from contexts where the client can't be freed safely, i.e. from the
  * lower level functions pushing data inside the client output buffers. */
 void asyncCloseClientOnOutputBufferLimitReached(client *c) {
+    if (c->fd == -1) return; /* It is unsafe to free fake clients. */
     serverAssert(c->reply_bytes < SIZE_MAX-(1024*64));
     if (c->reply_bytes == 0 || c->flags & CLIENT_CLOSE_ASAP) return;
     if (checkClientOutputBufferLimits(c)) {