Fix one-off error in big integers found by UBSan with fuzzer

[alexey-milovidov]

Changelog category (leave one):

• Bug Fix (user-visible misbehavior in an official stable release)

Changelog entry (a user-readable short description of the changes that goes to CHANGELOG.md):

Fix one-off error in converting a floating point number to big integers. It happens when one of the not least significant 64-bit parts of the resulting big integer is between 18446744073709550592 and 18446744073709551615. This closes #29039.

[robot-ch-test-poll1]

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