The btree data structure poisons regions of memory it's not using. It leaves
these regions poisoned when it frees memory. This means that a custom memory
allocator that tries to reuse freed memory will trigger an ASAN
use-after-poison error.

The fix is to unpoison each memory region right before freeing it.

PiperOrigin-RevId: 475309671
Change-Id: I29d55c298d3d89a83e1f960deb6e93118891ff83