Avoid object names with '//' to avoid hash inconsistencies (#8946)

This is to fix a situation where an object name incorrectly
is sent with '//' in its path heirarchy, we should reject
such object names because they may be hashed to a set where
the object might not originally belong because, this can
cause situations where once object is uploaded we cannot
delete it anymore.

Fixes #8873

cmd/object-api-input-checks.go

@@ -162,6 +162,7 @@ func checkObjectArgs(ctx context.Context, bucket, object string, obj ObjectLayer
 	if err := checkObjectNameForLengthAndSlash(bucket, object); err != nil {
 		return err
 	}
+
 	// Validates object name validity after bucket exists.
 	if !IsValidObjectName(object) {
 		return ObjectNameInvalid{