Filament monitor gives bad calibration data when filament not moving

[dc42]

If a print is started but the filament is not moving e.g. because it is stuck or not loading, after 10mm of commanded extrusion the print is paused as expected; however M591 reports stupid calibration values e.g. "inf" mm/rev for a magnetic filament monitor and a huge percentage. It would be better for the firmware to report zero or very little measured extrusion as "filament not moving" or similar.

This was observed using RRF 3.5.0-rc.1+ but is believed to be the case for earlier firmware versions too.