Crash in available_image_bounds error code path

An LLM-based code review claims that there is a bug here:

> composable.cpp:79 — available_image_bounds writes *error_status without a null check; the parameter defaults to nullptr. gap->available_image_bounds() crashes.

Could we get a volunteer to verify if this is a real bug? It sounds plausible, and could be easily verified by writing a test case and/or a test OTIO file that creates the scenario described.

Start at composable.cpp:79 and inspect available_image_bounds, especially its default error_status argument and the gap->available_image_bounds() call. Create the smallest test case or test OTIO file that exercises the described scenario, then confirm whether it crashes; done means the behavior is verified and, if reproducible, covered by a regression test.

Written by the indexing model from the issue text.