feefrac test: avoid integer overflow (bugfix) #32240

The feefrac_mul_div fuzz test fails after #30535 with the following (base64) input: Nb6Fc/97AACAAAD/ewAAgAAAAIAAAACAAAAAoA== (see https://cirrus-ci.com/task/5240029192126464?logs=ci#L3353).

This is caused by an internal multiplication inside CFeeRate that just exceeds the limit of the int64_t type. Fix that by tightening the bounds slightly further.

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For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/32240.

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cc @sr-gi

Copilot reviewed 1 out of 1 changed files in this pull request and generated no comments.

<details> <summary>Comments suppressed due to low confidence (1)</summary>

src/test/fuzz/feefrac.cpp:223

• The change from non-strict to strict inequalities correctly prevents borderline overflow conditions. Please verify that these adjustments do not inadvertently exclude valid cases that were intended to be processed.

if (mul64 < std::numeric_limits<int64_t>::max() / 1000 &&

</details>

Thanks, Copilot.

ACK a2bc330da86b604069308f5237b77cfa77ed2b43

Can confirm it was allowing one-too-large values for the computed CAmount, and that it fails on master.

utACK a2bc330da86b604069308f5237b77cfa77ed2b43

Trying to reproduce on my local setup, which is proving challenging (macOS).

I'll report later if I'm able to trigger it

ACK a2bc330da86b604069308f5237b77cfa77ed2b43, was able to reproduce + verify this fix

post-merge ACK, was able to reproduce locally and verify it doesn't crash anymore after the patch