refuse to activate a UTXO snapshot if mempool not empty

This ensures that we avoid any unexpected conditions inherent in
transferring non-empty mempools across chainstates.

Note that this should never happen in practice given that snapshot
activation will not occur outside of IBD, based upon the height checks
in `loadtxoutset`.

src/validation.cpp

@@ -5185,6 +5185,14 @@ bool ChainstateManager::ActivateSnapshot(
         return false;
     }
 
+    {
+        LOCK(::cs_main);
+        if (Assert(m_active_chainstate->GetMempool())->size() > 0) {
+            LogPrintf("[snapshot] can't activate a snapshot when mempool not empty\n");
+            return false;
+        }
+    }
+
     int64_t current_coinsdb_cache_size{0};
     int64_t current_coinstip_cache_size{0};