fix nil pointer deref in cli/upload.BlobCmd

[ansemjo]

I was playing around with the cosign cli and found that calling cosign upload blob ... with an image address but without any -f arguments leads to a nil pointer dereference. This patch adds a check that at least one file was provided.

The underlying cause is that calling cremote.UploadFiles with an empty files list never sets the var img v1.Image within and thus the returned dgstr is nil, I believe?

[dlorenc]

Thanks for the fix!

[ansemjo]

Sure! I wasn't sure if it was the right place to raise an error in BlobCmd or if dgster.Digest was more appropriate .. I force-pushed another edit using errors.New per your suggestion.

[dlorenc]

Thanks!