Fix a crash for abc-rpc test.

Summary: Revert an involuntary change made to CreateNewBlock() introduced by D249.

Test Plan: run make check and qa/pull-tester/rpc-tests.py

Reviewers: deadalnix, freetrader, kyuupichan, awemany, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D258