fix fits_update_value to work when there is no comment
fix fits_update_value to work when there is no comment

olebole/astrometry.net,olebole/astrometry.net,olebole/astrometry.net,olebole/astrometry.net,olebole/astrometry.net,olebole/astrometry.net,olebole/astrometry.net,olebole/astrometry.net