[qa] fix dbcrash and calculate_fee

Summary: Fix fee estimation bug which caused dbcrash to fail.

Test Plan:

./test/functional/test_runner.py --extended

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1926