[PATCH] Fix mutex pretty printer test and pretty printer output.

[Martin Galvan]

Thanks a lot for doing this; I was about to start working on something similar.

If I might make a suggestion: "Owner ID (if known)" doesn't really
help the user understand what's happening when the mutex is acquired
but the owner isn't recorded. I'd say we should at least document this
case somewhere (perhaps in the README?). I can write that myself if
you want to.