Fix several potential issues found by sanitizers

Summary: Backport of core PR9743

Test Plan:
With ubsan:

make check

With tsan:

./src/test/test_bitcoin --run_test=DoS_tests

(this test uses the SetMockTime function)

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2422