Do not panic on early tx broadcasts in fuzzing

Do not panic on early tx broadcasts in fuzzing

If the user broadcasts a funding transaction before the
counterparty provides a `funding_signed` we will panic in
`check_get_channel_ready`. This is expected - the user did
something which may lead to loss of funds, and we *really* need to
let them know.

However, the fuzzer can do this and we shouldn't treat it as a bug,
its a totally expected panic. Thus, we disable the panic in fuzz.

Thanks to Chaincode for providing fuzzing resources which managed
to hit this panic.