Autolinks to non-existent nodes cause problems in the router

If an autolink is created to a nonexistent node in a running container, a number of problems occurs:

• There is a connection error upon the closing of the session for the link, "no such channel", which causes the connection to close. This is particularly troublesome if there are other successful autolinks operating on the connection.
• When the connection closes, there can be a dereferencing of an invalid pn_link_t pointer, causing a segfault.
• The rate at which the connection is re-established and subsequently closed is very high, bounded only by available CPU.