vc_screen: fix null-ptr-deref in vcs_notifier() during concurrent vcs_write

A KASAN null-ptr-deref was observed in vcs_notifier():

BUG: KASAN: null-ptr-deref in vcs_notifier+0x98/0x130
Read of size 2 at addr qmp_cmd_name: qmp_capabilities, arguments: {}

The issue is a race condition in vcs_write(). When the console_lock is
temporarily dropped (to copy data from userspace), the vc_data pointer
obtained from vcs_vc() may become stale. After re-acquiring the lock,
vcs_vc() is called again to re-validate the pointer. If the vc has been
deallocated in the meantime, vcs_vc() returns NULL, and the while loop
breaks (with written > 0). However, after the loop, vcs_scr_updated(vc)
is still called with the now-NULL vc pointer, leading to a null pointer
dereference in the notifier chain (vcs_notifier dereferences param->vc).

Fix this by adding a NULL check for vc before calling vcs_scr_updated().

Fixes: 8fb9ea65c9d1 ("vc_screen: reload load of struct vc_data pointer in vcs_write() to avoid UAF")
Cc: stable@vger.kernel.org
Signed-off-by: Yi Yang <yiyang13@huawei.com>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Link: https://patch.msgid.link/20260604060734.2914976-1-yiyang13@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>