We preemptively perform a block mutation check before further processing
a block message (similar to early sanity checks on other messsage
types). The main reasons for this change are as follows:
- `CBlock::GetHash()` is a foot-gun without a prior mutation check, as
the hash returned only commits to the header but not to the actual
transactions (`CBlock::vtx`) contained in the block.
- We have observed attacks that abused mutated blocks in the past, which
could have been prevented by simply not processing mutated blocks
(e.g. https://github.com/bitcoin/bitcoin/pull/27608).
49257c0304