Merge bitcoin/bitcoin#27921: fuzz: Avoid OOM in transaction fuzz target

fa31c4d fuzz: Avoid OOM in transaction fuzz target (MarcoFalke)

Pull request description:

  To test: `FUZZ=transaction /usr/bin/time -f '%Us %MkB' ./src/test/fuzz/fuzz ../btc_qa_assets/fuzz_seed_corpus/transaction/9dc22b51df0af05ee5a595beefb0ce291feb6b99`

  Before: `0.72s 249636kB`
  After: `0.30s 92128kB`

ACKs for top commit:
  dergoegge:
    utACK fa31c4d

Tree-SHA512: 958fc54e7af31af7db3e3e1fb37553ae24de251c7fdeea3d68ec168f03db48de6aa54a96bf971f9cc804e94ff8a02fda9c56d7e85869d62962f6f020568e3a7b

Author: fanquake

src/test/fuzz/transaction.cpp

@@ -101,7 +101,14 @@ FUZZ_TARGET_INIT(transaction, initialize_transaction)
     (void)AreInputsStandard(tx, coins_view_cache);
     (void)IsWitnessStandard(tx, coins_view_cache);
 
-    UniValue u(UniValue::VOBJ);
-    TxToUniv(tx, /*block_hash=*/uint256::ZERO, /*entry=*/u);
-    TxToUniv(tx, /*block_hash=*/uint256::ONE, /*entry=*/u);
+    if (tx.GetTotalSize() < 250'000) { // Avoid high memory usage (with msan) due to json encoding
+        {
+            UniValue u{UniValue::VOBJ};
+            TxToUniv(tx, /*block_hash=*/uint256::ZERO, /*entry=*/u);
+        }
+        {
+            UniValue u{UniValue::VOBJ};
+            TxToUniv(tx, /*block_hash=*/uint256::ONE, /*entry=*/u);
+        }
+    }
 }