watchtower/wtdb: check tower exists on session create

Before creating a new session, first check that the TowerID that the
ClientSession is referencing refers to an existing tower. This is done
to prevent the situation where RemoveTower is called right before
CreateClientSession is called which would, before this commit, lead to
the session being created with a tower ID that does not refer to any
existing tower.

watchtower/wtdb/client_db.go

@@ -574,6 +574,11 @@ func (c *ClientDB) CreateClientSession(session *ClientSession) error {
 			return ErrUninitializedDB
 		}
 
+		towers := tx.ReadBucket(cTowerBkt)
+		if towers == nil {
+			return ErrUninitializedDB
+		}
+
 		// Check that  client session with this session id doesn't
 		// already exist.
 		existingSessionBytes := sessions.NestedReadWriteBucket(
@@ -583,7 +588,13 @@ func (c *ClientDB) CreateClientSession(session *ClientSession) error {
 			return ErrClientSessionAlreadyExists
 		}
 
+		// Ensure that a tower with the given ID actually exists in the
+		// DB.
 		towerID := session.TowerID
+		if _, err := getTower(towers, towerID.Bytes()); err != nil {
+			return err
+		}
+
 		blobType := session.Policy.BlobType
 
 		// Check that this tower has a reserved key index.