Fix panic on receiving `channel_ready` after 1st commitment update

Fix panic on receiving `channel_ready` after 1st commitment update

`cur_counterparty_commitment_transaction_number` starts at
`INITIAL_COMMITMENT_NUMBER`, gets decremented once when the initial
`channel_ready` message is received, and gets decremented a second
time when the first `revoke_and_ack` is received, revoking the
first counterparty commitment point.

At this point, `counterparty_prev_commitment_point` points to the
non-revoked second commitment point.

If we then process a second `channel_ready`, we check the
`cur_counterparty_commitment_transaction_number` number, see that
if is `INITIAL_COMMITMENT_NUMBER - 2` (i.e. not `- 1`) and assume
that the *second* commitment number has been revoked (by
`expect`ing `CounterpartyCommitmentSecrets::get_secret` with
`INITIAL_COMMITMENT_NUMBER - 1`). This `expect` panic's.

As the second commitment point has not yet been revoked, we should
fetch it from `counterparty_prev_commitment_point`, which we do
here, adding a test which failed on the previous code as well.

Found by the `full_stack_target` fuzzer.