Fix unlikely but possible race in `WeakValueDictionary` It may happen that value in `valueArray` could have been already collected by the GC but #clearDeadSlots have not yet been called. When this happened, `#at:ifAbsentPut:` returned tombstone rather than updating the dictionary with value from block. This commit fixes this by checking whether `valueArray` contain the tombstone and if so, clearing up the dead slots and restarting the operation. HTH.

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