This bug is a classic case of being lazy:
1. peer_accept_channel() allocated its return off the input message,
rather than taking an explicit allocation context. This concealed the
lifetime nature of the return.
2. The context for sanitize_error was the error itself, rather than the
more obvious tmpctx (connect_failed does not take).
The global tmpctx removes the "efficiency" excuse for grabbing a random
object to use as context, and is also nice and explicit.
All-the-hard-work-by: @ZmnSCPxj
044705a547