prevent traceback when root device is not defined #rhbz883768
prevent traceback when root device is not defined #rhbz883768

root defined as None was causing tracebacks when attempts to
determine root device's properties were run later in sanityCheck

resolves: rhbz#883768

vojtechtrefny/blivet,vpodzime/blivet,jkonecny12/blivet,dwlehman/blivet,rhinstaller/blivet,AdamWill/blivet,vojtechtrefny/blivet,vpodzime/blivet,rhinstaller/blivet,dwlehman/blivet,jkonecny12/blivet,rvykydal/blivet,rvykydal/blivet,AdamWill/blivet