internal/rosa: drain optional unstructured input

This was missed, causing non-minimal kindEtc IR to be rejected.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/rosa/etc.go

@@ -107,6 +107,8 @@ func init() {
 		a := cureEtc{}
 		if r.ReadUint32() == 0 {
 			a.minimal = true
+		} else {
+			r.Next()
 		}
 		if _, ok := r.Finalise(); ok {
 			panic(pkg.ErrUnexpectedChecksum)