fix coinbase detection for z inputs

lib/wallet.py

@@ -654,7 +654,7 @@ class Abstract_Wallet(PrintError):
                     return addr
 
     def add_transaction(self, tx_hash, tx):
-        is_coinbase = tx.inputs()[0]['type'] == 'coinbase'
+        is_coinbase = len(tx.inputs()) and tx.inputs()[0]['type'] == 'coinbase'
         with self.transaction_lock:
             # add inputs
             self.txi[tx_hash] = d = {}