tipc: fix nullptr crash during subscription cancel

tipc: fix nullptr crash during subscription cancel

commit 4d5cfcba2f6e ('tipc: fix connection abort during subscription
cancel'), removes the check for a valid subscription before calling
tipc_nametbl_subscribe().

This will lead to a nullptr exception when we process a
subscription cancel request. For a cancel request, a null
subscription is passed to tipc_nametbl_subscribe() resulting
in exception.

In this commit, we call tipc_nametbl_subscribe() only for
a valid subscription.

Fixes: 4d5cfcba2f6e ('tipc: fix connection abort during subscription cancel')
Reported-by: Anders Widell <[email protected]>
Signed-off-by: Parthasarathy Bhuvaragan <[email protected]>
Acked-by: Jon Maloy <[email protected]>
Signed-off-by: David S. Miller <[email protected]>