Digi-->cluster-->track chain doesn't always check that input collection is there

[bryngemark]

This is an old thing. There is no check in the cluster producer that input digits are actually there, causing it to segfault on occasion. There has also been a confusing case where these checks fill even if the collection is there, which has led me to at times not return in case the check fails. This can be seen in the track producer.

Clearly this is not the right way to do it. We should check, and skip processing events where needed input is missing.

The path ahead is, I think, to implement all these checks, and then test running on many input files to really probe the behavior.