Untrack key based on old->hasembkey (valkey-io#3007)

In `dbSetValue()` the `old` pointer may be reassigned to point to the
incoming value object which was created without an embedded key, so
calling `dbUntrackKeyWithVolatileItems()` would call `objectGetKey()`
which returns NULL, causing a crash in `hashtableSdsHash()` when trying
to hash the NULL key.

Idea is to assign `old_was_hash_with_volatile` before the swap and use
`new` instead of `old` for untracking when theres no embedded key.

Introduced in valkey-io#3003 
Run with NULL ptr dereference:
https://github.com/valkey-io/valkey/actions/runs/20701343184/job/59424029880

---------

Signed-off-by: Daniil Kashapov <daniil.kashapov.ykt@gmail.com>
Co-authored-by: Ran Shidlansik <ranshid@amazon.com>

Author: dvkashapov

src/db.c

@@ -373,7 +373,10 @@ static void dbSetValue(serverDb *db, robj *key, robj **valref, int overwrite, vo
 
     /* If overwriting a hash object, un-track it from the volatile items tracking if it contains volatile items.*/
     if (old->type == OBJ_HASH && hashTypeHasVolatileFields(old)) {
-        dbUntrackKeyWithVolatileItems(db, old);
+        /* Some commands create a new value (with NO key) and use setKey to change the value of an existing key.
+         * In this case the old can be replaced with the provided value and be left without a key
+         * however it is still a hashObject with optional volatile items and we need to untrack it. */
+        dbUntrackKeyWithVolatileItems(db, old->hasembkey ? old : new);
     }
     /* If the new object is a hash with volatile items we need to track it again */
     dbTrackKeyWithVolatileItems(db, new);