Some invalid input may be parsed so that the length of an opaque subpacket turns out to be 0. In such cases, arrange for a StructuralError to be returned indicating truncation. Found using gofuzz. Fixes golang/go#11503 Change-Id: Ib9ce8c604f35a31f852adfcd56a22dfd143a9443 Reviewed-on: https://go-review.googlesource.com/12634 Reviewed-by: Adam Langley <agl@golang.org>