grubby-bls: check if entry exists before attempting to print its info
    
    The grubby --info option didn't check if an entry existed for a kernel and
    printed wrong information about it. Fix this and also for --update-kernel
    and --set-default options that have the same issue.
    
    Resolves: rhbz#1634712
    
    Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>