Merge bitcoin/bitcoin#32240: feefrac test: avoid integer overflow (bugfix)

a2bc330 feefrac test: avoid integer overflow (bugfix) (Pieter Wuille)

Pull request description:

  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.

ACKs for top commit:
  sr-gi:
    utACK a2bc330
  instagibbs:
    ACK a2bc330
  glozow:
    ACK a2bc330, was able to reproduce + verify this fix

Tree-SHA512: cfbcdc8becfd518f4349ddc00c9af3ed0a23bb9534af71cc21df167d7038e5967127e5d97c4b3e8aeff6bf071c4f630c32ffaf81d8ec227954d21fdcbe205333

Author: glozow

src/test/fuzz/feefrac.cpp

@@ -220,9 +220,9 @@ FUZZ_TARGET(feefrac_mul_div)
         assert(res == res_fee);
 
         // Compare approximately with CFeeRate.
-        if (mul64 <= std::numeric_limits<int64_t>::max() / 1000 &&
-            mul64 >= std::numeric_limits<int64_t>::min() / 1000 &&
-            quot_abs <= arith_uint256{std::numeric_limits<int64_t>::max() / 1000}) {
+        if (mul64 < std::numeric_limits<int64_t>::max() / 1000 &&
+            mul64 > std::numeric_limits<int64_t>::min() / 1000 &&
+            quot_abs < arith_uint256{std::numeric_limits<int64_t>::max() / 1000}) {
             CFeeRate feerate(mul64, (uint32_t)div);
             CAmount feerate_fee{feerate.GetFee(mul32)};
             auto allowed_gap = static_cast<int64_t>(mul32 / 1000 + 3 + round_down);