Fix GH-16337: Use-after-free in SplHeap

We introduce a new flag to indicate when a heap or priority queue is
write-locked. In principle we could've used SPL_HEAP_CORRUPTED too, but
t

Fix GH-16337: Use-after-free in SplHeap

We introduce a new flag to indicate when a heap or priority queue is
write-locked. In principle we could've used SPL_HEAP_CORRUPTED too, but
that won't be descriptive to users (and it's a lie too).

Closes GH-16346.

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