Out of bounds panics when processing catalyst callback messages

[mjh1]

Related to #130

When I tested the extra MP4 outputs in the status messages i saw these panics:
https://eu-metrics-monitoring.livepeer.monster/grafana/explore?orgId=1&left=%7B%22datasource%22:%22P8E80F9AEF21F6940%22,%22queries%22:%5B%7B%22refId%22:%22A%22,%22datasource%22:%7B%22type%22:%22loki%22,%22uid%22:%22P8E80F9AEF21F6940%22%7D,%22editorMode%22:%22code%22,%22expr%22:%22%7Bapp%20%3D~%20%5C%22staging-task-runner%5C%22%7D%20%7C%3D%20%5C%22panic%5C%22%22,%22queryType%22:%22range%22%7D%5D,%22range%22:%7B%22from%22:%221675335647235%22,%22to%22:%221675337744323%22%7D%7D

The panic happens here, it seems this function assumes there will only ever be one output element?