p2p: Restrict downloading of blocks for snapshot chain

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.

src/net_processing.cpp

@@ -1397,6 +1397,15 @@ void PeerManagerImpl::FindNextBlocksToDownload(const Peer& peer, unsigned int co
         return;
     }
 
+    // When we sync with AssumeUtxo and discover the snapshot is not in the peer's best chain, abort:
+    // We can't reorg to this chain due to missing undo data until the background sync has finished,
+    // so downloading blocks from it would be futile.
+    const CBlockIndex* snap_base{m_chainman.GetSnapshotBaseBlock()};
+    if (snap_base && state->pindexBestKnownBlock->GetAncestor(snap_base->nHeight) != snap_base) {
+        LogDebug(BCLog::NET, "Not downloading blocks from peer=%d, which doesn't have the snapshot block in its best chain.\n", peer.m_id);
+        return;
+    }
+
     if (state->pindexLastCommonBlock == nullptr) {
         // Bootstrap quickly by guessing a parent of our best tip is the forking point.
         // Guessing wrong in either direction is not a problem.