SciFiHelicalPRTrack::_chisq_dof is sometimes NaN and gets written to the json output as a nan

"chisq_dof":-nan

When one tries to use this as an input, the JSON parser thinks nan is a string, but since it's not encapsulated within quotes, the decoding fails.

See Issue #1453 and see attached example of offending json

If the fit fails or if n_points is 0, chisq_dof should be set to some nonsense value instead of letting it remain not-a-number
Should probably also check if there are other potential NaN sources