Fix implicit-integer-sign-change in bloom

Summary:
This is a backport of core#24219
Depends on D12818

Test Plan:
With UBSAN
ninja && ninja check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12819