Fix crash during SLAVEOF when clients are blocked on lazyfree (#13853)

After https://github.com/redis/redis/pull/13167, when a client calls
`FLUSHDB` command, we still async empty database, and the client was
blocked until the lazyfree completes.

1) If another client calls `SLAVEOF` command during this time, the
server will unblock all blocked clients, including those blocked by the
lazyfree. However, when unblocking a lazyfree blocked client, we forgot
to call `updateStatsOnUnblock()`, which ultimately triggered the
following assertion.

2) If a client blocked by Lazyfree is unblocked midway, and at this
point the `bio_comp_list` has already received the completion
notification for the bio, we might end up processing a client that has
already been unblocked in `flushallSyncBgDone()`. Therefore, we need to
filter it out.

---------

Co-authored-by: oranagra <oran@redislabs.com>

src/blocked.c

@@ -270,6 +270,7 @@ void disconnectAllBlockedClients(void) {
 
             if (c->bstate.btype == BLOCKED_LAZYFREE) {
                 addReply(c, shared.ok); /* No reason lazy-free to fail */
+                updateStatsOnUnblock(c, 0, 0, 0);
                 c->flags &= ~CLIENT_PENDING_COMMAND;
                 unblockClient(c, 1);
             } else {

src/db.c

@@ -702,7 +702,7 @@ void flushallSyncBgDone(uint64_t client_id) {
     client *c = lookupClientByID(client_id);
 
     /* Verify that client still exists */
-    if (!c) return;
+    if (!(c && c->flags & CLIENT_BLOCKED)) return;
 
     /* Update current_client (Called functions might rely on it) */
     client *old_client = server.current_client;

tests/unit/lazyfree.tcl

@@ -174,4 +174,18 @@ start_server {tags {"lazyfree"}} {
         assert_equal [s lazyfreed_objects] 2
         $rd close
     }
+
+    test "Unblocks client blocked on lazyfree via REPLICAOF command" {
+        set rd [redis_deferring_client]
+
+        populate 50000 ;# Just to make flushdb async slower
+        $rd flushdb
+        wait_for_blocked_client
+        # Test that slaveof command unblocks clients without assertion failure
+        r slaveof 127.0.0.1 0
+        assert_equal [$rd read] {OK}
+        $rd close
+        r ping
+        r slaveof no one
+    } {OK} {external:skip}
 }