Fix #75673: SplStack::unserialize() behavior

Even though `SplStack::unserialize()` is not supposed to be called on
an already constructed instance, it is probably better if the method

Fix #75673: SplStack::unserialize() behavior

Even though `SplStack::unserialize()` is not supposed to be called on
an already constructed instance, it is probably better if the method
clears the stack before actually unserializing.

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