wallet: Re-introduce change assertion for withdrawals

This was removed because withdraw-to-self results in more than the
change being sent to us.

Signed-off-by: Christian Decker <decker.christian@gmail.com>

wallet/walletrpc.c

@@ -61,6 +61,7 @@ static void wallet_withdrawal_broadcast(struct bitcoind *bitcoind,
 
 		/* Note normally, change_satoshi == withdraw->changesatoshi, but
 		 * not if we're actually making a payment to ourselves! */
+		assert(change_satoshi >= withdraw->changesatoshi);
 
 		struct json_result *response = new_json_result(cmd);
 		json_object_start(response, NULL);