Negative values of relative humidity in CFSR
I am also getting negative relative humidity from the GFS (Global Forecast System): 36-hour predicted atmosphere dataset (Provider: NOAA/NCEP/EMC, their website: http://www.emc.ncep.noaa.gov/index.php?branch=GFS)

I found this link that answers as to why relative humidity is possible in their dataset: 

http://ftp.cpc.ncep.noaa.gov/wd51we/reanal/random_notes/negative.h2o

This might apply to the one you have access too. 

Let me know if this helps.