drbd: Add NULL check for net_conf to prevent dereference in state validation

drbd: Add NULL check for net_conf to prevent dereference in state validation

If the net_conf pointer is NULL and the code attempts to access its
fields without a check, it will lead to a null pointer dereference.
Add a NULL check before dereferencing the pointer.

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 44ed167da748 ("drbd: rcu_read_lock() and rcu_dereference() for tconn->net_conf")
Cc: stable@vger.kernel.org
Signed-off-by: Mikhail Lobanov <m.lobanov@rosalinux.ru>
Link: https://lore.kernel.org/r/20240909133740.84297-1-m.lobanov@rosalinux.ru
Signed-off-by: Jens Axboe <axboe@kernel.dk>