Remove redundant NULL checks after new
Summary:
Partial backport of Core PR10898 (commit 55224af)
https://github.com/bitcoin/bitcoin/pull/10898/commits/55224af6bdecf79bb0d2c95b16f1ff60b16c45f8
Test Plan: ninja check
Reviewers: deadalnix, Fabien, #bitcoin_abc
Reviewed By: Fabien, #bitcoin_abc
Subscribers: teamcity, schancel
Differential Revision: https://reviews.bitcoinabc.org/D2622