[tabgab]

Fixes #1434. On master 7ec0d4d, one commit.

A node can keep an interface outside its interface table, for example in a submodule with an interface table of its own (BridgingRouter in tests/module/lib, the Simu5G Upf). The topology has no destination interface for a link that ends at such an interface, and GateScheduleConfiguratorBase::addPorts dereferenced the null pointer.

Change

1. addPorts: once the remote node is known, a link with no destination interface is skipped with an EV_WARN that names the local interface and the remote node. Its port stays without endNode and otherPort, exactly like the port of an unconnected interface.
2. addFlows: the search for the output port of a flow ignores ports without an endNode, and a flow whose path needs such a link stops with

   Cannot find an output port with a connected destination interface from node 'client' to node 'extra' on the path of a flow
   

   instead of dereferencing the end iterator.

The two belong together: skipping the link in addPorts makes the dereference in addFlows reachable.

Why skip in addPorts and not stop

The link is legitimate, and the schedule of the TSN part usually does not involve it. The error is raised where it matters, for a stream whose path needs the link, and it names both nodes.

Eager and AlwaysOpen reach ports only through the paths that addFlows builds, and Z3 reads endNode and otherPort only through those paths (its loop over all ports reads the datarate only), so they are covered. TSNschedGateScheduleConfigurator reads port->endNode of every switch port (line 91), which is null for an unconnected switch port on master as well; it is not changed here.

Evidence

test (tests/module)	case	master	with the fix
GateScheduleConfigurator_2.test	link to an unregistered interface, stream elsewhere	segfault (exit 139)	runs to the time limit
GateScheduleConfigurator_3.test	stream client -> extra over that link	segfault (exit 139)	exit code 1 and the message above

• With the companion PR (guard band of a datarate-0 interface), both patches apply with git am without conflict, and the three new GateScheduleConfigurator_* tests pass together.
• showcases/tsn/gatescheduling/eager, config Eager, runs to its time limit with the fix.
• Release build, macOS arm64, OMNeT++ 6.5.0.

Not run: debug build; the full tests/module and fingerprint suites; Z3 and TSNsched (not built here); a real Simu5G Upf network against this INET; Linux and Windows.