Document assumptions that are being made to avoid NULL pointer dereferences

Summary:
Backport of Core PR11132
Completes T519
Depends on D2320

Test Plan:
ninja check
make check

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2321