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: [e-mail]
Signed-off-by: Mikhail Lobanov <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jens Axboe <[e-mail]>

drivers/block/drbd/drbd_state.c

···
 		  ns.disk == D_OUTDATED)
 		rv = SS_CONNECTED_OUTDATES;
 
-	else if ((ns.conn == C_VERIFY_S || ns.conn == C_VERIFY_T) &&
+	else if (nc && (ns.conn == C_VERIFY_S || ns.conn == C_VERIFY_T) &&
 		 (nc->verify_alg[0] == 0))
 		rv = SS_NO_VERIFY_ALG;