T101705: Don't create a lazy socket for dangling connections

In the commit db0ef8b00d3c there was a standardized behavior for reroute dangling nodes.
But that only took into account the connection. Lazy sockets in the constructor of multisocket
were still being created. This resulted in an error when looking up their standard
value, as there was no input. This patch adds dangling connection checking logic
to the multisocket constructor as well.

Also, this looks like a temporary workaround for now, as the dangling data can
be cached to avoid creating nodes of unused reroutes. But for now, it's enough
just to have a method to check for dangling.