validation: don't modify genesis during snapshot load

Avoid modifying the genesis block index entry during snapshot load. This
is because, in a future change that fixes LoadBlockIndex for UTXO
snapshots, we detect block index entries that are reliant on
assumed-valid ancestors and treat them specially.

Since the genesis block doesn't have BLOCK_VALID_SCRIPTS, it would be
erroneously marked BLOCK_ASSUMED_VALID during snapshot load if we didn't
skip it here. This would cause a "setBlockIndexCandidates() empty"
assertion to be tripped since all block index entries would be marked
assume-valid due to genesis, which is never re-validated.

There's probably no good reason to modify the genesis block index entry
during snapshot load anyway...

src/test/validation_chainstatemanager_tests.cpp

@@ -232,6 +232,9 @@ BOOST_FIXTURE_TEST_CASE(chainstatemanager_activate_snapshot, TestChain100Setup)
         *chainman.ActiveChainstate().m_from_snapshot_blockhash,
         *chainman.SnapshotBlockhash());
 
+    // Ensure that the genesis block was not marked assumed-valid.
+    BOOST_CHECK(!chainman.ActiveChain().Genesis()->IsAssumedValid());
+
     const AssumeutxoData& au_data = *ExpectedAssumeutxo(snapshot_height, ::Params());
     const CBlockIndex* tip = chainman.ActiveTip();