Cluster: don't update slave's master if we don't know it.
Cluster: don't update slave's master if we don't know it.

There is no way we can update the slave's node->slaveof pointer if we
don't know the master (no node with such an ID in our tables).

JackieXie168/redis,JackieXie168/redis,JackieXie168/redis,JackieXie168/redis