fix(electrum): prevent fetch_prev_txout from querying coinbase transactions

\`fetch_prev_txout\` should not try to query the prevouts of coinbase
transactions, as it may result in the Electrum server returning an error
which would cause the overall `sync` to fail.

Author: LagginTimes

crates/electrum/src/bdk_electrum_client.rs

@@ -422,7 +422,7 @@ impl<E: ElectrumApi> BdkElectrumClient<E> {
     ) -> Result<(), Error> {
         let mut no_dup = HashSet::<Txid>::new();
         for tx in &tx_update.txs {
-            if no_dup.insert(tx.compute_txid()) {
+            if !tx.is_coinbase() && no_dup.insert(tx.compute_txid()) {
                 for vin in &tx.input {
                     let outpoint = vin.previous_output;
                     let vout = outpoint.vout;