adb.add_transaction: try to ser-deser tx early

Previously calling add_transaction with a malformed Transaction obj could
result in an exception late in the flow, after the walletdb was already side-effected.
Rollback of such side-effects is not implemented :/
  but this small patch should at least cover and prevent some common cases.

```
File "/opt/electrum/electrum/address_synchronizer.py", line 358, in add_transaction
  self.db.add_transaction(tx_hash, tx)
File "/opt/electrum/electrum/json_db.py", line 42, in wrapper
  return func(self, *args, **kwargs)
File "/opt/electrum/electrum/wallet_db.py", line 1434, in add_transaction
  tx = tx_from_any(str(tx))
File "/opt/electrum/electrum/transaction.py", line 1339, in tx_from_any
  raise SerializationError(f"Failed to recognise tx encoding, or to parse transaction. "
```

electrum/address_synchronizer.py

@@ -31,7 +31,7 @@ from .crypto import sha256
 from . import bitcoin, util
 from .bitcoin import COINBASE_MATURITY
 from .util import profiler, bfh, TxMinedInfo, UnrelatedTransactionException, with_lock, OldTaskGroup
-from .transaction import Transaction, TxOutput, TxInput, PartialTxInput, TxOutpoint, PartialTransaction
+from .transaction import Transaction, TxOutput, TxInput, PartialTxInput, TxOutpoint, PartialTransaction, tx_from_any
 from .synchronizer import Synchronizer
 from .verifier import SPV
 from .blockchain import hash_header, Blockchain
@@ -275,6 +275,8 @@ class AddressSynchronizer(Logger, EventListener):
         tx_hash = tx.txid()
         if tx_hash is None:
             raise Exception("cannot add tx without txid to wallet history")
+        # For sanity, try to serialize and deserialize tx early:
+        tx_from_any(str(tx))  # see if raises (no-side-effects)
         # we need self.transaction_lock but get_tx_height will take self.lock
         # so we need to take that too here, to enforce order of locks
         with self.lock, self.transaction_lock: