If the best chain of the peer doesn't include the snapshot
block, it is futile to download blocks from this chain,
because we couldn't reorg to it. We'd also crash
trying to reorg because this scenario is not handled.
This is a partial backport of core#29519
https://github.com/bitcoin/bitcoin/pull/29519/commits/7a885518d57c6eb818ebef5fd04a575f324ee8b2