Modify secrets pkg

Made a mistake in my earlier patch. I though that if you add an empty string
to an array, the length of the array would still be 0...
Realised this when vendoring the secrets pkg into cri-o.

Signed-off-by: umohnani8 <umohnani@redhat.com>

Closes: #685
Approved by: mheon

libpod/options.go

@@ -194,7 +194,10 @@ func WithDefaultMountsFile(mountsFile string) RuntimeOption {
 			return ErrRuntimeFinalized
 		}
 
-		rt.config.DefaultMountsFile = []string{mountsFile}
+		if mountsFile == "" {
+			return ErrInvalidArg
+		}
+		rt.config.DefaultMountsFile = mountsFile
 		return nil
 	}
 }