Fix a minor memory leak on PermanentFailure mon errs when sending

Fix a minor memory leak on PermanentFailure mon errs when sending

If we send a payment and fail to update the first-hop channel state
with a `PermanentFailure` ChannelMonitorUpdateErr, we would have an
entry in our pending payments map, but possibly not return the
PaymentId back to the user to retry the payment, leading to a (rare
and relatively minor) memory leak.