validation: don't try to invalidate genesis block

Summary:

The code for creating the candidate block map assumes that the passed block's previous block (pindex->pprev) is available and otherwise segfaults due to null-pointer deference in CBlockIndexWorkComparator(). Rather than analyzing all possible code paths and implications for this corner case, simply fail early if the genesis block is passed.

This is a backport of core#20921

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12264