Don't create chan-closed mon update for outbound never-signed chans

Don't create chan-closed mon update for outbound never-signed chans

Like the previous commit for channel-closed monitor updates for
inbound channels during processing of a funding_created message,
this resolves a more general issue for closing outbound channels
which have sent a funding_created but not yet received a
funding_signed.

This issue was also detected by full_stack_target.

To make similar issues easier to detect in testing and fuzzing, an
additional assertion is added to panic on updates to a channel
monitor before registering it.