Well, you are right that hints are not initialized for dummy slot. However, it is not a correctness issue. Because, 1) hints are used to distribute contention in the starvation-resistent queue, the actual value is not really important, and 2)slots in arena object are initilized by memset first and then by arena::process when a worker joins, the dummy slot can probably be used only by master thread in some situations - not a problem.
Are you rather concerned about some warning by the compiler or a correctness tool?