#3359 ntf: fix ntfd coredump

[Khuong Ba Le]

Coredump happens when try to access to a null pointer in ntfd.

BT ###
#0 0x0000558e5506a000 in NtfClient::getMdsDest() const ()
#1 0x0000558e55072213 in NtfAdmin::processNotification(unsigned int, SaNtfNotificationTypeT, ntfsv_send_not_req*, mds_sync_snd_ctxt*, unsigned long long) ()
#2 0x0000558e55072487 in NtfAdmin::notificationReceived(unsigned int, SaNtfNotificationTypeT, ntfsv_send_not_req*, mds_sync_snd_ctxt*) ()
#3 0x0000558e5505805f in ?? ()
#4 0x0000558e55058844 in ?? ()
#5 0x0000558e55058c2b in ntfs_process_mbx ()
#6 0x0000558e55056637 in main ()

Step Reproduce:
1. Simulate nfs hang to make the ntf buffer full.
2. Client send many notifications to server.
3. Reboot all client to simulate client down while sending ntf.
* Recommend: using a cluster with multiple node payloads to simulate numerous clients sending notifications simultaneously. This will increase the likelihood of overload, thereby raising the chances of encountering a core dump.

[Thien Minh Huynh]

commit eb24877a84905aa5d72b9551764c271c8b3bd24e (HEAD -> develop, origin/develop)
Author: khuonglb khuong.b.le@endava.com
Date: Wed Oct 1 15:51:31 2025 +0700

ntfd: prevent coredump on null client during notification [#3359]

ntfd could crash when a notification was processed after client was removed.
Leaving a null client pointer and causing a null pointer dereference

Check for a null client before handling notifications and return early.