refactor: Avoid UB from std::distance with nullptr in PoolAllocator #34161

1. 
   l0rinc commented at 1:01 pm on December 28, 2025: contributor

   Problem

   Calling std::distance(nullptr, nullptr) is undefined behavior per the C++ standard iterator.requirements.general:

   Iterators can also have singular values that are not associated with any sequence. Results of most expressions are undefined for singular values.

   Null pointers are singular iterators not associated with any sequence, so they cannot form a valid range required by std::distance:

   Preconditions: last is reachable from first

   While expr.add#5.1 defines pointer subtraction for null pointers:

   If P and Q both evaluate to null pointer values, the value is 0

   This applies to direct pointer arithmetic (nullptr - nullptr), not to std::distance, which has additional preconditions requiring non-singular iterators.

   Fix

   Checked for nullptr before calling std::distance to avoid this undefined behavior. This check only affects the very first call, after which the pointers are initialized.

2. refactor: Avoid UB from `std::distance` with `nullptr` in `PoolAllocator`

   Calling `std::distance(nullptr, nullptr)` is undefined behavior per the C++ standard [iterator.requirements.general](https://eel.is/c++draft/iterator.requirements.general#7):
   > Iterators can also have singular values that are not associated with any sequence. Results of most expressions are undefined for singular values.
   
   Null pointers are singular iterators not associated with any sequence, so they cannot form a valid range required by [std::distance](https://eel.is/c++draft/iterator.operations#4):
   > Preconditions: last is reachable from first
   
   Check for `nullptr` before calling `std::distance` to avoid this UB. This check only affects the very first call, after which the pointers are initialized.

   b9ffd11eec
3. DrahtBot added the label Refactoring on Dec 28, 2025
4. 
   DrahtBot commented at 1:01 pm on December 28, 2025: contributor

   The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

   Code Coverage & Benchmarks

   For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/34161.

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5. l0rinc marked this as ready for review on Dec 28, 2025

Contributors
l0rinc DrahtBot

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Refactoring