Skip memory prefetch during loading to avoid crash in dictEmpty callback (#14848)

Fixes #14838

## Summary

Fix a crash in `prefetchCommands()` that occurs during replica full sync
when the replica has existing data that needs to be emptied.

## Problem Description

During `emptyData()` → `kvstoreEmpty()` → `dictEmpty()` →
`_dictClear()`, the first hash table is cleared and `d->ht_table[0]` is
set to NULL via `_dictReset`. Then while clearing the second hash table,
every 65536 buckets it invokes `replicationEmptyDbCallback()` →
`processEventsWhileBlocked()` → `readQueryFromClient()` →
`prefetchCommands()`.
At this point, `dictSize() > 0` still holds (because the second hash
table isn't fully cleared yet), but `ht_table[0]` is already NULL. The
prefetch code assumed `ht_table[0]` is always valid when `dictSize() >
0`, leading to a crash.

## Solution
1. **Skip prefetch during loading**: Added a `server.loading` check at
the top of `prefetchCommands()` to return early. During RDB loading, the
main dictionary is being rebuilt, so prefetching keys from it is useless
anyway.
2. **Add defensive assertion**: Added
`serverAssert(batch->current_dicts[i]->ht_table[0])` in
`initBatchInfo()` to catch any future cases where `ht_table[0]` is NULL
while `dictSize() > 0` (which should only happen mid-`dictEmpty` via
`_dictReset`).

---------

Co-authored-by: kairosci <kairosci@users.noreply.github.com>
Co-authored-by: debing.sun <debing.sun@redis.com>
Co-authored-by: Yuan Wang <yuan.wang@redis.com>

src/memory_prefetch.c

@@ -165,6 +165,11 @@ static void initBatchInfo(dict **dicts, GetValueDataFunc func) {
             info->state = PREFETCH_DONE;
             continue;
         }
+
+        /* We skip prefetch during loading, so ht_table[0] should never be NULL
+         * when dictSize() > 0 (which only happens mid-dictEmpty via _dictReset). */
+        serverAssert(batch->current_dicts[i]->ht_table[0]);
+
         info->ht_idx = HT_IDX_INVALID;
         info->current_entry = NULL;
         info->current_kv = NULL;
@@ -334,7 +339,7 @@ int determinePrefetchCount(int len) {
  * 3. Prefetch the keys and values for all commands in the current batch from
  *    the main dictionaries. */
 void prefetchCommands(void) {
-    if (!batch) return;
+    if (!batch || server.loading) return;
 
     /* Prefetch argv's for all pending commands */
     for (size_t i = 0; i < batch->pcmd_count; i++) {

tests/integration/replication.tcl

@@ -1832,3 +1832,43 @@ start_server {tags {"repl external:skip"}} {
         }
     }
 }
+
+start_server {tags {"repl external:skip"}} {
+    set master [srv 0 client]
+    set master_host [srv 0 host]
+    set master_port [srv 0 port]
+
+    start_server {overrides {io-threads 2}} {
+        set slave [srv 0 client]
+
+        test {prefetchCommands handles NULL argv and keys during RDB replication with IO threads} {
+            # Enable diskless sync to trigger RDB streaming during replication
+            $master config set repl-diskless-sync yes
+            $master config set repl-diskless-sync-delay 0
+
+            # Populate keys in the format key:$i with 128-byte values.
+            $slave debug populate 700000 key 128
+
+            # Force a full resync by resetting the slave.
+            set rd [redis_deferring_client 0]
+            $rd slaveof $master_host $master_port
+
+            # Create a large pipeline command.
+            set batch_size 1000
+            set buf ""
+            for {set i 0} {$i < $batch_size} {incr i} {
+                append buf [format_command get key:1]
+            }
+            
+            # Continuously send pipelined commands so that the replica processes
+            # and prefetches them while it is emptying old data during full sync.
+            set start_time [clock milliseconds]
+            while {[clock milliseconds] - $start_time < 5000} {
+                $rd write $buf
+                $rd flush
+                if {[s 0 master_link_status] eq "up"} break
+            }
+            $rd close
+        }
+    }
+}