Fix sendpayment without amount argument, BIP21 only

Fixes bug introduced with 83c6bc053a

scripts/sendpayment.py

@@ -96,7 +96,7 @@ def main():
                 sweeping = True
             destaddr = args[2]
         mixdepth = options.mixdepth
-        if btc.is_bip21_uri(args[2]):
+        if len(args) > 2 and btc.is_bip21_uri(args[2]):
             parsed = btc.decode_bip21_uri(args[2])
             if 'amount' in parsed:
                 parser.error("Specify amount as a separate argument or amount in BIP21 URI, not both.")