Sanity assert GetAncestor() != nullptr where appropriate

Summary:
Add sanity asserts for return value of CBlockIndex::GetAncestor() where appropriate.

In validation.cpp CheckSequenceLocks, check the return value of tip->GetAncestor(maxInputHeight) stored into lp->maxInputBlock. If it ever returns nullptr because the ancestor isn't found, it's going to be a bad bug to keep going, since a LockPoints object with the maxInputBlock member set to nullptr signifies no relative lock time.

In the other places, the added asserts would prevent accidental dereferencing of a null pointer which is undefined behavior.

Co-Authored-By: Aurèle Oulès <aurele@oules.com>
Co-Authored-By: danra <danra@users.noreply.github.com>

This is a backport of core#24804

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13242