Acquire cs_main before ATMP call in block_assemble bench

Otherwise we fail an assert in sync.cpp:AssertLockHeldInternal.

src/bench/block_assemble.cpp

@@ -97,11 +97,15 @@ static void AssembleBlock(benchmark::State& state)
         if (NUM_BLOCKS - b >= COINBASE_MATURITY)
             txs.at(b) = MakeTransactionRef(tx);
     }
+    {
+        LOCK(::cs_main); // Required for ::AcceptToMemoryPool.
+
         for (const auto& txr : txs) {
             CValidationState state;
             bool ret{::AcceptToMemoryPool(::mempool, state, txr, nullptr /* pfMissingInputs */, nullptr /* plTxnReplaced */, false /* bypass_limits */, /* nAbsurdFee */ 0)};
             assert(ret);
         }
+    }
 
     while (state.KeepRunning()) {
         PrepareBlock(SCRIPT_PUB);