Merge #8665: Assert all the things!

Summary:
4d51e9b Assert ConnectBlock block and pIndex are the same block (NicolasDorier)
972714c pow: GetNextWorkRequired never called with NULL pindexLast (Daniel Cousens)
cc44c8f ContextualCheckBlockHeader should never have pindexPrev to NULL (NicolasDorier)

Tree-SHA512: 7cc568bf9417267c335f21ec3d1505b26e56e5b3d5f4d3dbb555279489800aaa65a3bcd7bc376e274dd102912aec16ddbb18de2e2060b2667b41eb979cd9321e

Backport of Core PR 8665
https://github.com/bitcoin/bitcoin/pull/8665/files

Test Plan:
make check
test_runner.py

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2722