[io] Fix an asan buffer overflow detection in TBufferFile.

[hageboeck]

As a followup to fe1be25, increase the auto-allocated array buffer by one element. In fe1be25 (PR #22165), the external allocation in TBasket (size == fNevBufSize) was replaced with an automatic allocation in TBufferFile::ReadArray(Int_t), whose size was the number of elements read from the file.
However, when ReadArray(Int_t) is used as an offset array, it is assumed to contain one more element than visible from the size field stored on disc. See here:

root/tree/tree/src/TBasket.cxx

Lines 1250 to 1279 in da08f46

	if (entryOffset) {
	bool hasOffsetBit = fIOBits & static_cast<UChar_t>(TBasket::EIOBits::kGenerateOffsetMap);
	if (!CanGenerateOffsetArray()) {
	// If we have set the offset map flag, but cannot dynamically generate the map, then
	// we should at least convert the offset array to a size array. Note that we always
	// write out (fNevBuf+1) entries to match the original case.
	if (hasOffsetBit) {
	for (Int_t idx = fNevBuf; idx > 0; idx--) {
	entryOffset[idx] -= entryOffset[idx - 1];
	}
	entryOffset[0] = 0;
	}
	fBufferRef->WriteArray(entryOffset, fNevBuf + 1);
	// Convert back to offset format: keeping both sizes and offsets in-memory were considered,
	// but it seems better to use CPU than memory.
	if (hasOffsetBit) {
	entryOffset[0] = fKeylen;
	for (Int_t idx = 1; idx < fNevBuf + 1; idx++) {
	entryOffset[idx] += entryOffset[idx - 1];
	}
	}
	} else if (!hasOffsetBit) { // In this case, write out as normal
	fBufferRef->WriteArray(entryOffset, fNevBuf + 1);
	}
	if (fDisplacement) {
	fBufferRef->WriteArray(fDisplacement, fNevBuf + 1);
	delete[] fDisplacement;
	fDisplacement = nullptr;
	}
	}

The auto-allocated buffer needs to be large enough to cover this case, so here we propose to generally over-allocate the buffer by one int.

To give some numbers from this particular case:
Before #22165, the offset array buffer was sized using fNevBufSize, 1000 in this case.
After the PR, it was 84, but we would have needed 85 elements to fit it.
What we propose is to overallocate all buffer for ReadArray(Int_t), leading to generally smaller buffers for offset arrays, and to one additional int for arrays that are not used as offset arrays.

Thanks to @silverweed for helping to investigate.

[github-actions]

Test Results

    23 files      23 suites   3d 15h 37m 16s ⏱️
 3 872 tests  3 866 ✅   0 💤 6 ❌
79 695 runs  79 584 ✅ 105 💤 6 ❌

For more details on these failures, see this check.

Results for commit 9c994b2.

♻️ This comment has been updated with latest results.

[pcanal]

That seems to be a fixed need to one particular case of the use of ReadArray that is costing (a bit) every other use of ReadArray (apriori there are many distinct ones). I/we would have to look at the particular case (offset array) and see if there isn't any better (aka better localized) solution.