Assert in MoviewPipelinePIEExecutor when cancelling movie scenes during rendering

When rendering a single Level Sequence with MRQ, removing the Level Sequence that is actively rendering will create an array out of bounds exception.

Also confirmed in //UE5/Main, CL: 36086643

1. Download the attached test project and open
2. Open the Level Sequence "/Game/LS_Crashy"
3. Render the sequence using MRQ
4. While the render is active, remove the sequence from the queue.
5. Observe the check() for array index out of bounds.

Note to QA: This isn't specific to the provided level sequence. Any queue will work just fine to try the repro.