Fix #80663: Recursive SplFixedArray::setSize() may cause double-free

We address the `::setSize(0)` case by setting `array->element = NULL`
and `array->size = 0` before we destroy the ele

Fix #80663: Recursive SplFixedArray::setSize() may cause double-free

We address the `::setSize(0)` case by setting `array->element = NULL`
and `array->size = 0` before we destroy the elements.

Co-authored-by: Tyson Andre <tyson.andre@uwaterloo.ca>

Closes GH-7503.

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